I've been experiencing this frustrating problem with my desktop. It will randomly turn off, i.e. completely power down without any errors on screen.
Things i've done:
I've had a look at the event viewer and there isn't anything that stands out.
Opened the case to check all the fans are spinning when powered on.
Checked to make sure everything is well seated into the Mobo.
Re-applied themal paste to the processor in case it was overheating.
Swapped out my 450W PSU with a 500W PSU i had.
Not sure what else to try.
The problem began roughly when i upgraded my gfx card but i have since put my old card back in but i'm still getting the problem.