...did you recently uninstall a DVD player such as PowerDVD or similar? (Uninstalling DVD players can occasionally remove the DVD codecs from the machine.)

Try downloading and installing the free VLC Media Player from the link below:

http://www.videolan.org/vlc/

And last but not least, if the DVD drive is old, it's possible it's starting to fail. Does the DVD disc play on another DVD player? If so, and if you've tried using a different player which contains the right DVD codecs, then it may be time to replace the drive.

Hope this helps.

Grif