Have you installed the lastest drivers for your graphics card? If you haven't, then follow this steps:
- Click on the Dash
- Search for Additional and click Additional Drivers
- Wait a bit, and you'll see a list with all the compatible drivers for your computer.
- Check if your graphics card name appears at that list, select the entry for it (I usually select the "post-release" entry, or the one that has [Recommended]) and click Activate. After a while, you'll have to restart your computer.
Maybe this will work to you, be sure to have all the updated drivers before of doing anything! Hope it works to you and tell us soon.
(Sorry for the duplicated answer, the original by me can be find here.)