Solution for Cannot adjust display brightness

The problem is caused by Windows incorrectly identifying the notebook's screen.

To fix, open the Device Manager, expand 'Monitors', select your monitor, right-click it and select Uninstall. Then, click the 'Scan for Hardware Changes button. You should now be able to adjust the display's brightness again!

Source

http://windowsforum.com/threads/cant-adjust-display-brightness.5924/#post-242751

Solution for Cannot adjust display brightness