My car battery went dead.

First check for a blown fuse. There will be two for the radio, but the memory fuse is tied to some other circuit that is always live, typically the interior lights or horn. Fuses often blow when jump-starting or reconnecting the battery from the current surges of the many computer memory circuits charging up.

If there's no blown fuses, it's time to visit the dealer. They will either need to reprogram the radio or read a code that has to be translated, then the new code installed.