My desktop suddenly stopped when i was doing my activity today. I couldn't do anything, including switching to virtual terminal (i thought it was KDE who stucked, but actually it's the whole system). I had to press the POWER button to turn it off and boot it again. Again, it happened after i login to KDE for a few minutes.
I thought it was the hard disk that has a problem, so i checked my computer part by part. The hard disk was fine, so my next target was my RAM. I took my RAM off and clean it before i plug it again. After that, i clean up the dust inside my casing and testing it again and yeah... it works like normal again.
This is just a tip, but sometimes it works. Usually, if your system is locked up, check your RAM or Power Supply. This two components have the highest probability of being the source of the problem.