1996 Cadillac Deville radio locked

Sounds like you have one of those theft lock radios to me, must be the battery went dead or or for some reason the battery power was lost to the radio, they are designed to do that so the radio is usless to a thief, you will have to get the reset code and type it in using the preset buttons, you might be able to call the dealer and with proper id you can get that code, if you can't then other option is to take car into a dealer and they will have to reset it for you record the code somewhere and if it happens again you can do it yourself