Is the security light flashing when you try to start the car? If so, try performing a "Passlock Relearn":
- Put the key in the ignition and attempt to start the car, the "Security" light should go on and the car won't crank. Don't move the key to the "off", leave it on.
- Leave the key in the on position for around 10 minutes, the security light should eventually go out.
- Turn the key to "off", wait 5 seconds, then attempt to start the car. If it starts, you're done! If it doesn't start, to back to step 1 and try again. You may need to repeat this process up to 3 more times to get the car started.
Some things to note:
- Try to turn off any lights / equipment in the car while you're doing this to avoid killing the battery.
- This is just a temporary fix and the problem will usually come back. The permanent solution usually requires replacing the sensor in the lock cylinder and possibly getting a fresh key made by the dealer (not a locksmith, you want an authentic GM key from a GM/Chevrolet dealer - they'll make Pontiac keys). I had this problem in my 2001 Monte Carlo, which shares a lot of parts with the Grand Prix, and I had to replace both to get the problem to go away.