My PC is "turning off"?

This issue happened to me about 3 or 4 months ago but it's happening again today, at a random time, my monitor, mouse, and keyboard turn off and it seems like my pc is off but my tower is on and running fine. I can't turn off my pc with the power button either I have to hit the switch on my outlet. It went away before i'm not sure when it can possibly be. Maybe video drivers? bleh i don't know.. help? suggestions??
 
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