Most of the time, it's the laser-lens that burns out after a while. Also, the stepper motor of the CD drive may be damaged, but that's only a guess.
First, inspect the area around the laser lense and the lense itself and clean it with a alcohol-soaked q-tip. Pay attention not to use too much force, also make sure that the game CDs you're using are okay, clean them with a soft dry cloth if necessary, then try again.
If it still doesn't work, you will have to inspect it or sent it to someone for inspection. The neostore used to sell Neo CD Repair Kits back then (CD reader unit, spare lense), not sure wether they're still available, just check the store or email Shawn.
Also, check out the links in
this thread for more info.