Did it just suddenly stop working or get worse over time? Have you added or taken away any other hardware from your computer? Is it a new drive you just fitted?
Possible probems could be:-
Dirty lens on laser. Try a CD Laser cleaning disk.
Loose connections inside the case (Turn off power before looking)
If its a new drive, or other hardware has been added or removed, there may be some configuring you need to do, or jumpers that need to be moved... check with the documentation that came with the drive.
Hope this helps a bit, sorry if it doesn't.
Mark L., June 2004