logout issue in ubuntu 12.10

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http://askubuntu.com – I have installed ubuntu 12.10 after installation every things worked fined 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 my laptop power button once 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 happens. my laptop is hp probook 4520s . (HowTos)