Frist thing to do is to put in a few original CDs that you have bought. This will make sure that it isn't a single CD.
Next have you installed any new software just before it stopped working?
Also have you done anything "silly:p"to the drive. I.E flashing the firmware.
Have you moved/ banged the computer and a cable has come out? (This is always a good on to look for, at least it is on my machine)
Have ago with those few things first. If it's still not working then we will have to give windows the once over.