Hi
pogomwg.
A good first step in troubleshooting a recalcitrant app is to delete its preferences.
Every application creates a preferences file which is stored in /Users/[yourusername]/Library/Preferences. These are named according to the convention com.softwaredeveloper.applicationname.plist. Sometimes, they can become corrupted and cause the application to malfunction.
So try this: quit DVD Player. Go to your Home folder, look in Library for the folder called Preferences, and locate the file called com.apple.DVDPlayer.plist. Delete it and restart DVD Player.
I can't say this will definitely fix your problem, but it's a good general troubleshooting technique to know anyway.
If this doesn't help, you might try downloading
VLC, which is a highly regarded free media player. If your DVDs don't play under VLC either, that might suggest you have a hardware problem. VLC is nice to have around anyway, as it can play some media files that Quicktime balks at.
Hope this helps.