I’ve an ATI graphics card. When going away for a while, Kubuntu could lock up with a black screen when the powersave mode of the graphics card/monitor kicked in.
This is a known, as far as I know issue with ATI drivers.
I could reboot the system cleanly throught the magic keys ALT- BUSIER mnemonic, but it was pretty annoying. The XOrg log show something like: [fglrx] ASIC hang happened.
Doing this, sudo aticonfig –auto-powerstates=off –effective=startup solved the issue so far..