2000 Chevy Cavalier

Does it plug into the a/c compressor clutch?Do you need to buy one of these connector.

No that is for the a/c nothing to do with it running. SO what's going on with not running?Does it crank no start or no crank no start?The more detail's the better.

So is the security light still flashing or staying on solid?

Give this a try>30 Minute Learn Procedure
TOOLS REQUIRED:
None

1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON (vehicle will not start).
3. Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF
4. Turn OFF the ignition, and wait 5 seconds.
5. Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes (the vehicle is now ready to relearn the Passlock(TM) Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK).

IMPORTANT: The vehicle learns the Passlock (TM) 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(TM) 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).

You need to turn the key to the start position then let it sit at the run position till the security light turn's off then repeat that 2 more time's without turning the key off in between before the light turn's off. If you replace the ignition like you said then this what it's going to take to get it running again.