only can suggest a reset of it.. unplug and leave for a while, then reconnect... some punto stereos need a code, some don't... no ryhme or reason behind what one you got... (my one doesn't need a code, my dad's did)... if it appears to be working but asks for a code then go through a garage and they'll send it off to get the code for you or you could try through the dealership that first sold the car as they may have taken a record of it (v unlikely though)...
if not spend a 100 quid and get a cd player to replace it... plenty of reasonably quality basic cd stereos around for that...