ubuntu - Screen doesn't turn on sometimes, after dual boot installation

I have Windows 8 and installed Ubuntu 13.10, as dual boot. After this installation, the screen doesn't turn on sometimes, even though the system loads normally in the background (I hear the welcoming sound of the OS). A hard reset is required in order for the screen to turn on.

These "sometimes", precisely, are:

• When I cold boot the computer.


• When I am running one of the OSs and I restart the computer, so I can load the other OS.