If you changed the car battery or your car had some electrical problems, it is likely that car radio cassette or car radio CD player to get locked and ask for a password or code to unlock. 

If you don't have the right code, provided with the cd player or radio cassette player, do not try to guess the code, because you can get your car player locked forever.

Is highly recommended to contact your auto dealer or brand representative and ask them for the unlock code. If you provide them the necessary data, they will give you the unlock code.