My Becker PCM has stopped working!?

My Porsche Boxster 3.2s (02) had been left standing for a couple of weeks in cold weather. I came to start it and the PCM screen flickered & then went blank. However the radio, cd & cassette player still worked.
Also in some bright conditions I could faintly see the Sat Nav screen & also if I pressed the sat nav option the voice came on saying 'your satellite navigation is ready' (or a similar message!) I was advised to try disconnecting the batter and then re-connecting, I have tried this and now I have nothing, no radio or cd etc!

Has anyone any suggestions for fixing this problem please?