It's in theft mode. Manufacturer's anti-theft systems are real effective at keeping owners out of their own cars, and GM's are some of the best. First try unlocking the front doors with the key. If that doesn't turn the system off, you can try the relearn procedure:
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 Passlock Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK).
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 desired (history DTCs will self clear after 100 ignition cycles).
If that doesn't work, suspect a problem with the ignition switch assembly or the sensor in it.
Saturday, October 27th, 2012 AT 9:04 PM