Assuming you have determined it is not to do with sleep or hibernation settings, have you tried shutting the screen saver down to see if it might be creating the problem. If this is an issue that started recently you might want to try restoring the system to an earlier time.
On a few occasions just recently my win 7 pro system has come out of "screen saver" with nothing but my desktop background displayed. If I then ctrl/alt/delete the screen goes black with just the cursor showing but otherwise unuseable. After a while the background will reappear but the only way that I can get the icons and everything back is to reboot.
I have done a very thorough search for malware but none found. The problem never occurs whilst I am actually using the system and it does not always happen when it has been left to go into screen saver.
Any one any ideas?