Debbie_tim, the short answer is that one of the two or three lasers in your drive has likely burned out, since it still turns otherwise. Optical drives now last about a year, but some with heavy use will burn out after six months or less.
The easiest solution (unless this is a laptop) is to replace the DVD burner. They are relatively inexpensive now... running as low as $24.99 and as high as $129... but you can get an excellent LG or Samsung for under $40. Avoid the Sony, HLDS, HP, ASUS, because they do not seem to last as long. The Plextors and Yamaha drives are much more reliable and long lasting, but they cost a lot more as well.
You can find drives online at various sites such as directron, tigerdirect, newegg, pcconnection, zipzoomfly, cdw, and many others, or you can pop down to your friendly neighborhood Wal-Mart, Best Buy, Circuit City, or Target.