Ok, this may require only a reset.
Take the battery cables off the battery, hold the ends together for 10 seconds then attach them to the battery and see what happens. This will reset the whole vehicle. If you have 2 batteries, then remove both and touch only one set.
After that,it may need a relearn of the security system.
Turn the key on and attempt to start the car, It may crank, it may start and stall or it may do nothing at all, this is fine, still cycle the key to start then release the key back to run. Wait 10 minutes with the key in run. The passlock light should be flashing and the DIC will say "Service Vehicle" or "Security" or have a picture of a car with a lock during that 10 minute span. After 10 minutes has passed, turn off the car, count to 10, and try to start it again and release the key back to run. Wait another 10 minutes. After 10 minutes, turn car off 10 seconds and try to start again and release the key to run. Wait another 10 minutes. After 10 minutes, turn car off 10 seconds and try to start again. It should start up this time. The passlock has been learned. During this process, turn the fans off, radio off and cycle the headlights off so the battery will not go dead. If needed, use a battery charger.