Marantz CDR-500 - Help

My Marantz CDR-500 Professional Dual CD Recorder is suddenly refusing to recognize the inserted blank CDR. A message keeps coming up stating "INSERT DISC" although the disc is already in place. I have tried several blank discs with the same result. What do you think is wrong and how can I fix it ?

Does it attempt to spin the disc after insertion, or do you not even hear a spinning ?
If it spins but does not read I'd guess the laser pickup has gone.
If it does not spin it could be the switch that detects that the drawer has closed has malfunctioned. I had this problem with my CD67, and it seems to have been fixed simply by removing the drawer front, allowing the drawer to retract a little further into the player.

Also take the lid of and look for debris (in my case cat hair) in the little white helical screw threads. This can cause the player to be unable to position the laser.