Kenwood RC520 Car Stereo

Stereo will not play. The only thing that will show on readout is ALL OFF. It may have been accidently recoded. How can I find the new code or recode it again myself? It has work fine for four years. I have checked all fuses, tryed reset, tried diconnecting all power to unit and still no results. Should mention that it worked fine until I removed the cassett from the CD player, replace it and nothing has worked sinse.
Thanks in advance for your time and help. Richard