I have had something happen twice and hope someone can tell me what might be happening.
I am doing something (don't think it was on the internet...just with files). All at once my screen went dark. Not black, but gray. I wiggled the mouse nothing happened. There was no mouse pointer. I did ctrl/alt/backspace. Nothing happened. I did ctrl/alt/delete. Nothing happened. I then pushed the button on the tower. The blue desktop came up and the computer turned off. Before it happened my right hand was on the mouse and my left was on the keyboard. I don't think I did anything with my left hand.
This has happened twice but not on the same day...perhaps weeks apart. Everything else seems to be working OK.