|I'm assuming you have made no software changes. |
If this is a desktop computer, something inside the case can develop a poor connection when you physically move the case from one place to another, especially if you didn't move it yourself.
Open up the case and make sure the video card is all the way down in it's slot.
If the video card has one or two power sockets, make sure the connectors from the power supply are all the way down in that (those) sockets.
If the DVD is a movie, if the software codec that decodes the video is built into the ATI software, you will not be able to play the movie if the video chipset on the card is not being detected properly and because of the the ATI software is not loaded properly.
If there is nothing wrong with the DVD drive, you will be able to see the files on the DVD in Windows Explorer even if you can't play the files.
(All Programs - Accessories - Windows Explorer - scroll down, open up My Computer, click once on the drive letter for the drive the DVD is in.)
If the files on the DVD do not show up in Windows Explorer at all.....
Combo DVD rom / CD burner drives or combo DVD burner drives (read and burn both DVDs and CDs) have at least two lasers and associated circuits - sometimes one of those malfunctions after the dive has been used a lot.