I let my custom build pc render a video (for youtube) while i left for around 2 hours. During this time my pc shut down. I came back, pressed the power button, and nothing happened. I switched outlets, I heard fans spin up when i pressed the power but they died in a half second. To make a long stoy short, it wasn't the psu. Figured it was the mobo, and i was right. It was as if it just couldn't carry an electrical current. Luckily my friend had an extra mobo.
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Your mobo is dead some how. Get is for RMA if it has warranty.