Welcome to Tom's, please stop posting in all caps.
What OS are you using?
Pre-Vista OS's (like XP) do not contain software capable of playing DVD's. You will have to download or buy a suitable DVD decoder/player program. DVD drives come with with the software on a disk if you bought a retail DVD drive.
If you are running an OS Vista or later:
A. Your DVD decoder software is corrupted.
B. Your DVD drive is bad. (but most likely A)