The problem is that the anti-theft system for the stereo system has has gone into the lock mode to let would-be-theifs know that if they have slolen it to re-install in another vehicle it will not work. You need to call or go to your local G.M. dealer and give them your V.I.N. # located on the front driver side of your dash board, and they can tell you the code or reset the code for you, and hen it should work. This problem usually happens when the battery is disconnected for what ever reason , when re-connecting the battery cable, the stereo goes into this lock out mode. I have a friend who had the same problem with a 97' Cavalier, the dealership fixed it for him at no charge. Good luck with it.