Can't start my 1998 3.1 Monte Carlo

You have a Pass lock problem. There are 3 wires from the Ignition switch that reads the ignition key, if you have lot's of other keys on your key chain. That puts extra stress on the ignition lock switch, which in turns wear out the wires in the lock that reads the key.
The dealer will cost over $500.00 to replace the lock and key's to match. I got a good friend at a lock smith to over ride the system by running a diode across the 3 wires, but now i have no pass lock.
Try this, it might help if the wires are ok, it's to reset the pass lock that the dealer will try and still charge you hundred of dollars since they want to change the lock and key's for you.
Good luck and hope this helps: The relearn process is pretty simple and will take 30 minutes, if it'll work for you at all since your problem doesn't sound like a computer issue.

30-Minute Learn Procedure
1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON; the vehicle will not start.
3. Observe the SECURITY tell tale light. After approximately 10 minutes, the light will turn OFF.
4. Turn OFF the ignition switch, and wait 5 seconds.
5. Repeat steps 1 through 4 - two more times - for a total of 3 cycles of 10 minutes. 3x10 = 30 minutes.
The vehicle is now ready to relearn the Passlock Sensor Data Code and/or passwords on the next ignition switch turn from OFF to Crank.

Important:
Turn the Ignition Switch OFF.
The vehicle learns the Passlock Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6. Start the engine. The vehicle has now learned the Passlock Sensor Data Code and/or password.
7. With a scan tool, clear any DTCs if needed. History DTCs will self clear after 100 ignition cycles.
Without a scan tool you may have momentary starter interruptions while history clears itself.

When the security light is blinking that means the ECM thinks there is a security issue and has shut the car down to keep it from being stolen. Either someone tried to start it with a key without the chip in it there is just an issue with your security system. Do you have any information on events leading up to this problem?

If you have a tilt wheel and used quite often a break in the wire can cause this problem

Unless the modual is bad this will get you running again .turn all acc of like radio and a/c. when you turn the key it will be from off to on where the lights come on but not all the way to start.
1. turn they key to on leave it on wait 10 minutes and once the light falshes and goes out turn the key 3 times from off to on then leave on again 10 more minutes and again once it flashes and goes out turn the key 3 times off to on quickly then repeat again. you do this 3 times for 30 minute total after the last time and you turn the key 3 times go ahead and start the car. unless the mod is bad this will get you going .
1. turn key on leave on
2. in 10 minutes the light will flash and or go out as well turn turn the key from off to on 3 times
3. repeat this 3 times for 30 minutes and then start the car.