Sony 6 CD player in a 03 Mondeo estate not playing CDs?

I have the Sony 6 CD player fitted in my 03 Mondeo estate. When I load the CDs they will not play... get the message 'Check CD', tried a single CD as well as 6 CDs, I am using genuine CDs not copies. I bought the keys to extract the unit, disconnected power lead but still no joy. Any help will be much appreciated.