Ubuntu – Lock Screen Blank

I have recently upgraded to ubuntu 12.04. I have been having some trouble with lock screen lately. While the system is locked, I sometimes get blank screen without the lock screen. So I am unable to log back in. I have to shut down the system & restart once again. While this does not happen every-time, it nevertheless keeps happening from time to time. This usually happens when I leave the system running leaving the system to lock on its own. Can anyone suggest some measure. My keyboard shortcuts or manual lock screen are both working perfectly fine.