Go through and take note of your codecs, plugins and programs and then try doing a system restore back to when it worked. Compare plugins as well as codecs and programs and then undo the restore if need be. Also, WMP is not a stand alone DVD player- you might have known that already but just covering the basics. If you unistalled the DVD player that came with your drive or another one you installed, it may have taken the plugin that WMP is/was using to play the DVD.
Just some things to check.