Okay, for me to help you I have to ask you a few questions first
1.) Does your Computer (Laptop/Desktop) computer even have a DVD Drive?
2.) What DVD viewing software are you using (I.E. PowerDVD, Windows Media Player)
3.) Has the computer suffered a fall (A Physical Fall, Like dropping it)
4.) Is the DVD drive software installed?
One thing you should do first is go to device list to see if it is working correctly... Instructions below:
1.) Open the Control Panel
2.) From the control panel double left click on System
3.) On the top of the window there should be some tabs... Left click once on the "Hardware" tab
4.) Then click on Device List
If there is a problem with your hardware it should show up in the window that just opened... Please answer my above questions if you want more help...