OK, so either the PC or the monitor looks to be failing. I'm guessing you don't have it set to turn off the monitor in 10 minutes in Windows so for this round we need a PC and/of another monitor to see what's at fault.
Your story is still slim. You didn't tell if you found an entire load of cat hair and rodent droppings in the computer. Well I exaggerate. Any clue is better than none as I'm not there to see what you saw.