Was it working fine then stopped working?
Try this:
1) Open START->"Computer" to view Windows Explorer.
2) Now insert a disc.
3) Do you see a new drive (probably "D") with anything in it? (audio CD's will just show "tracks" and normal DVD-Video discs will show an "Audio" and "Video" folder).
You can also try a WINDOWS or GAME disc to see if anything pops up.