I have installed ubuntu 12.10 after installation every things worked fine and for 2 weeks I've had no problem . yesterday I tried to log off and log in again but when I logged off it showed me a black screen and nothing happend I've waited for 30 minutes but still it was black . I had to press and hold power button to shut down my laptop.
I've had this problem on ubuntu 12.04 too. from yesterday every time that I want to log off this issue is happening.
My laptop is hp probook 4520s .