Kdcbt42u protected mode problem

Refer to Sony KDL 55EX500 Troubleshooting Manual Protection Mode Symptom...
5X Blink - TCON & HFR Failure
The 5X blink protection mode indicates a communications error between the TCON/HFR board and the MT5388 microprocessor on the A-Board.
First replace the TCON/HFR Board if the 5X protection mode blink pattern occurs. If the TCON/HFR Board does not repair the problem then replace the A-Board.

see schematics in Sony KDL 55EX500 Troubleshooting Manual Page 23

It's in the protect mode because of a component failure inside. It needs to be repaired to get out of the protect mode.

Do you mean into the unprotected mode?, if so

Turning OFF Protect mode in a Riso MZ 790u (and other models I'm sure).

1. Go to Admin Tab on the main console screen and scroll to the "Protect" button.
2. Select (press) the "Protect" button on the screen; printer will go through a reset function (takes about 3-5 seconds).
3. After printer is finished "resetting", pull out one of the master disposal boxes (doesn't matter which one).
4. Press the "Protect" button again.
5. the screen should say "Protect = Off"
6. You're done and your machine should be un-locked now.

Access safe mode by rebooting and clicking on F8 key till safe screen is there select "boot with network enabled", select Administrator and find the section on protection mode.

That is a mode designed to protect the amplifier from damage.
Common causes of this are heat, incompatible Ohm level of speakers, or a short on the chassis or terminals.

most camera data cards have a 'write protect' slide switch on the side of them, which if it is in the 'write protect' mode, nothing can modify, or put new files onto it.
Basically what it's telling you is that you can't take pictures with that card until you pop it out and slide the switch to an 'unlock' or 'non-write-protect' position.

Protect mode will kicking whenever a fault is detected.

Causes that can start protect mode are:

Electrostatic harges on internal circuits.
Leave the unit unplugged for power for one hour or so, this will allow discharge.

Bad speaker wiring.
Check speaker wiring, try unplugging and plugging back, or start after removing one speaker at the time.

Internal fault.
Internal fault is the natural cause for protect mode. If all the above did not work, then the problem is a blown component or an internal fault. In this case the unit must be taken apart , tested and repaired by an experienced person.
If the unit is less than one year old, or if you want to get service, call Sony at the phone number listed on the product documentation.

Regards.

Turn on the car so the radio is flashing "Protect." Look on the front / removable face of the stereo. On it you will find a small round hole - that's a reset button. Find a small paper clip, unfold the clip and stick it gently into the hole. The "Protect" will stop flashing, and Kenwood will be displayed. Unfortunately all your settings/presets will be gone, but at least the stereo is working again!

Usually, protect mode is enabled when the speaker output wiring has shorted together or is touching ground/+12V. I would verify that none of the speaker wiring has come loose.