As Mucks said, really, my first thought was to try a CD drive cleaner (you can get 'em in Sainsburys, Maplins and other places for about six quid).
Try several different CD's in there as well.
I just bought one of those CD cleaners, as my audio system Marantz CD player was refusing to read some home made burnt CD's. It hadn't been cleaned since I bought it about 11 years ago.
After doing the clean cycle twice, when I opened the tray I noticed a lot of dust on the CD tray. It now reads all CD's OK and was obviously in severe need of a clean.