Stereo will change CDs, but they won't play..?

This is typical for CD changers. It is probably a ust buildup on the laser. The only way to stop it from happening is to clean the laser. To do this, you would have to remove the top cover, locate the laser and use a dry cotton swab to wipe it off. (this is a delicate component, so you cannot apply too much pressure or any liquids or you could damage it.) The best thing to do if you have no experience with electronic equipment is take it to a local service center and ask them to clean it. Good luck.