I've run into a strange problem lately. I recently installed Ubuntu 12.04 on my Lenovo Ideapad Y550 and have been installing the recommended updates regularly. Things were running smooth until last night when I found out that the screen freezes, and at times goes black, when I try to lock the screen from the system menu or by pressing Ctrl+Alt+L. The only option I have is to restart lightdm. I'm using the proprietary Nvidia driver on my laptop. To make matters worse, I couldn't shut down the computer either from the normal user account. It just took me to the login screen. So I had to press the power button to shut down the computer.
I've had the same problem here. I installed the video drivers from the software center. I actually think you need to go and install the fglrx drivers in addition to any proprietary drivers. But I haven't had 'as much' of a problem since doing that but it will still from time to time not display the desktop after being put to sleep. My only solutions has been rebooting. I had this problem on another laptop of mine and ended up reinstalling. It may have something to do with not installing 3rd party software at the time of OS install. I'll give re-installation a shot again and post my results.