Shift lock button broke off and the vehicle is locked in park and unable to shift into gear.

Something wrong with the shift lock system maybe. Safety system that won't allow the gear selector to move from park unless the driver is pushing the brake pedal . Do your brake light's work ? Does your caddy have floor or column shift ?
Automatic Transmission Shift Lock Control Description and Operation (Floor Shift)
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
• The automatic transmission shift lock solenoid (serviced as the automatic transmission shift lock actuator), as well as the body control module (BCM) and the engine control module (ECM).The shift lock solenoid is located within the floor shift control assembly with vehicles equipped with floor shift.
• The BCM controls the voltage supply circuit of the shift lock control solenoid. The following conditions must be before the BCM will supply voltage to the shift lock control solenoid.
The BCM controls the voltage to the shift lock control solenoid through the shift lock control solenoid controlled voltage circuit. The following conditions must be met before the BCM will supply voltage to the shift lock solenoid:
• The ignition is in the ON position.
• The ECM sends an input via GMLAN serial data to the BCM indicating the transmission is in the PARK position.
• The BCM determines the brake pedal is applied according the brake pedal position.
Since the shift lock control solenoid is permanently grounded, the BCM supplies voltage to the automatic transmission shift lock control solenoid, mechanically unlocking the shift lever allowing the driver to move the shift lever out of the PARK position as the solenoid energizes. When the brake pedal is not applied, the BCM turns the control voltage output of the shift lock control solenoid OFF, de-energizing the shift lock control solenoid. The de-energized solenoid mechanically locks the shift lever in the PARK position.
During remote start operation, the BCM will de-energize the shift lock control circuit, locking the shift lever in the PARK position.

possible internal locking pawl stuck fault or adjustment required

ADJUSTMENTS
sourced from another forum

The park interlock cable is part of the brake/shift lever interlock system. Correct cable adjustment is important to proper interlock operation. The gear shift and park lock cables must both be correctly adjusted in order to shift out of PARK.

ADJUSTMENT PROCEDURE

1. Remove floor console as necessary for access to the park lock cable.

2. Shift the transmission into the PARK position.

3. Turn ignition switch to LOCK position. Be sure ignition key cylinder is in the LOCK position. Cable will not adjust correctly in any other position.
NOTE: If the key will not turn to the LOCK position, pull up on the cable lock button and manually move the cable in and out until the key can be turned to the LOCK position.

4. Pull cable lock button up to release cable, if necessary.

5. Remove and discard the cable adjuster lock pin, if a new cable is being installed.

6. Ensure that the cable is free to self-adjust by pushing cable rearward and releasing.

7. Push lock button down until it snaps in place. The lock should be 1-2mm below the surface of the cylindrical portion of the cable

I am mechanic I really need to no more make ect. but it could be battery or the ignition switch or ignition lock n key but like I said need more information this is a guess that I told you

Vehicles have a safety system that won't allow gear selector to be moved out of park unless brake pedal is pushed ! This what Buick calls there's
Automatic Transmission Shift Lock Control ! Depending on make of your vehicle it mite be called something else . But all vehicles have it.
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the ignition is ON. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
?€¢
The automatic transmission shift lock control solenoid

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The automatic transmission shift lock control switch

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The brake to shift relay

?€¢
The rear integration module (RIM)

With the ignition in the ON position, battery positive voltage is supplied to the brake to shift relay coil. When the RIM energizes the brake to shift relay, the brake to shift relay switch contacts close suppling battery voltage to the automatic transmission shift lock control switch. The circuit continues through the normally closed switch to the automatic transmission shift lock control solenoid. The permanently grounded solenoid is energized. This locks the shift linkage in the PARK position. When the driver presses the brake pedal, the contacts in the automatic transmission shift lock control switch open. This causes the automatic transmission shift lock control solenoid to release. This allows the shift lever to move from the PARK position.

What make vehicle ? Most all vehicles have a safety system .Don't try an force anything you'll break it !
Electronic Park Lock (EPL)/Ignition Lock Cylinder Control Actuator: If the vehicle is equipped with automatic transmission and a floor mounted console gear shift, it has an ignition lock cylinder control actuator system in the steering column. The ignition lock cylinder control actuator purpose is to prevent the ignition key from being turned to the OFF position when the transmission is in any position other than PARK and the vehicle may still be moving. The column ignition lock system consists of an ignition lock cylinder control actuator, and a park position switch that is located in the automatic transmission shift lock control switch. The ignition lock cylinder control actuator contains a pin that is spring loaded to mechanically prevent the ignition key cylinder from being turned to the lock position when the vehicle transmission is not in the PARK position. If vehicle power is lost, and/or the transmission is not in the PARK position the operator will not be able to turn the ignition key to the lock position and will not be able to remove the ignition key from the column. They also have this on column mounted shifters !

Do you have column or floor shifter ?
All automatic transaxle models use an electric park lock system. Within the housing assembly, a lock cylinder is connected to an actuator which rotates and engages on the electric park lock solenoid. When the floor-shift lever is not in the park position, the actuator disengages from the electric park lock solenoid slider. This allows the lock cylinder to move to the lock position. With the shift lever in PARK and the lock cylinder in LOCK, the actuator cam is positioned against the electric park lock solenoid slider. This prevents the shift lever from shifting out of the PARK position.
The column-shift automatic transaxle model uses a flexible cable actuator instead of an electric park lock system. One end of the cable is attached to the gate assembly. The other end of the cable is attached to the column mounted housing assembly. This system functions similarly to the floor-shift ignition lock system.
The manual transaxle model uses a push button lock cylinder. A clutch start switch is used. The clutch pedal must be pressed before the engine will crank. Refer to Park Lock System Check .
CAN you turn the key on ?
The gear selector does not shift out of PARK with the brake pedal depressed and the ignition switch in RUN.
Automatic Transmission Shift Lock Control Always On with Ignition in Run
Action
Value(s)
Yes
No

1

Was the BTSI System Check performed?
--
Go to Step 2
Go to Automatic Transmission Shift Lock Control System Check

2

Is this a column shift vehicle?
--
Go to Step 3 yes
Go to Step 4 no

3

1. Move the gear selector to the PARK position.
2. Disconnect the automatic transaxle control connector
3. Turn the ignition switch to RUN.
4. Connect the test lamp between terminal B and ground.

Does the test lamp light?
--
Go to Step 4
Go to Step 8

4

1. Disconnect the brake switch connector C2.
2. Move the ignition switch to RUN.

Can you shift the transaxle out of the PARK position?
--
Go to Step 5
Go to Step 8

5

Is the brake switch adjusted properly?
--
Go to Step 6
Go to Step 7

6

Replace the brake switch.
Is the repair complete?
--
System OK
--

7

Adjust the brake switch.
Is the repair complete?
--
System OK
--

8

1. Disconnect the BTSI solenoid connector (column shift vehicles) or disconnect the automatic transaxle control connector (console shift vehicles).
2. Move the ignition switch to RUN.

Can you shift the transaxle out of the PARK position?
--
Go to Step 9
Go to Step 10

9

Repair the short to power in CKT 1135.
Is the repair complete?
--
System OK
--

10

Test for sticking or binding in the shift linkage.
Does the shift linkage stick or bind?
--
Go to Step 11
Go to Step 12

11

Repair the shift linkage.
Is the repair complete?
--
System OK
--

12

Replace the BTSI solenoid (column shift vehicles) or replace the automatic transaxle control (console shift vehicles).
Is the repair complete?
--
System OK
Go to Step 1

Shift lock control on this vehicle is complicated - your problem is probably a malfunctioning part in the system such as the shift lock control switch at the brake pedal, the Park relay, the shift lock relay (if equipped) or the A/T shift lock control solenoid. Take it to a dealer ...

get under car with jackstands-not a jack -and ajust linkage-u also may hav broke somethin-buy haynes manual at parts place

The park shift lock needs adjustment. The solenoid that locks irt inplace maybe the cause of this

The term "shift lock" is the system that is in place to prevent the vehicle from being shifted out of Park into Reverse or Drive WITHOUT having the driver's foot firmly on the brake pedal. The brake light switch on the brake pedal closes a circuit that energizes a solenoid under the shifter. When the brake is applied, and the solenoid is energized, the shift lever is no longer "locked", and can be moved to the desired position.
The idea here stems from a phenonomon several years ago that dealt with the issue of "Sudden Intended Acceleration". What was happening here was, for some unknown reason, engines started developing high RPMS for no reason, and several accisents were reported due to this issue. The vehicle would accelerate "unintendedly", when putting into drive or reverse, and could hit something.