It's kinda tough to troubleshoot a problem when we don't know much about your setup at all. We only know that you've got a Mac that's got a DVD drive in it and DVD Player won't work...
What exact model of Macintosh are you using? Do the DVDs you insert mount on the desktop and just won't play in DVD Player, or are they not recognized by the system at all? Did you make any changes to your system (hardware or software) before this happened?
If you can elaborate on your situation a little more, we can help to make a more thorough assesment of the problem.