1997 Buick Park Avenue radio

theres a code that you enter. its usually five digits. if you've had the car since new the dealer will have that info. sometimes its written in the owners manual

also make sure your fuse isn't blown. the clock face will be blank if the fuse is blown

If the Theft-Lock was never activated, it is more likely a fuse blew from the current surge when the new battery was connected. That is especially likely if there is no display lighting up. There will be two fuses to check. One is listed for the radio and one usually is not. The memory circuit is tied in with some other circuit that is always live like the interior lights, horn, or cigarette lighter.

If there is a display and the Theft-Lock was activated, you'll need to visit the dealer. They will press a combination of buttons to get a code out, then they call an 800 number to have that translated to the code that has to be punched in to turn the radio on. There is a way to change the code to all "0"s. That will turn the Theft-Lock off so this won't happen again. That system is mainly meant to make money for the dealers, not to prevent theft of the radio.