2000 Chevy Lumina

Do you have a security light on or flashing when you try to start it?

The Passlock system is locking you out. I'm giving you the relearn procedure. This may or may not work. If it doesn't, that means you have a hard failure in one of the other components and you will need a factory scan tool to diagnose it.

30 Minute Re-Learn Procedure
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 Passlockâ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK).
Important
The vehicle learns the Passlockâ 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 Passlockâ Sensor Data Code and/or password).
7. With a scan tool, clear any DTCs if desired (history DTCs will self clear after 100 ignition cycles).

That means you have a failure in the Passlock system and you will need a professional scan tool to be able to diagnose it. The dealer may be the best bet.