Have you checked all the cables are in place? Do the leds still light up, draw open+close, cd spin? If not, your cables aren't right.
Else, try 'add new hardware' in control panel.
Have you tried restarting with a boot-disk to see if your rig actually detects the drive? Using windows you can boot from floppy using cd-rom support. If you do this and don't get a message 'drive not found' then the drive itself is ok, and your cds may be knackered.
A quick re-install of windows usually sorts out these annoying glitches if all else fails.