Try a cd that you know works (right format etc.). If that doesnt work either take it to a shop and have them repair it or replace it. Sounds like something inside that switches the speakers isnt working. Thats just a guess, I have never had one apart and they dont send them with a wiring schematic so there you go.
first off i will ask what manufacturer it is and how old it is
i know we have had problems with kenwood and especially on their newer line of heads with them just burning up the motor that spins the disk
if you put a disk in you should hear it start spinning if it dosent that is probably what it is
if it does spin you can get a lense cleaner for mobile audio head units you can try that
if not if it is still under the manufacturer warinty you can take it back and get a new one if not you may have to buy one