My computer just won't turn on

So I had a friend over who installed a new power supply into my computer, before he left we turned on my computer and plugged my monitor in to make sure it was all okay. Everything checked out and he went home, then I unplugged the power and moved my computer back into my desk and plugged it back in. When I did it refused to turn on and no matter what I do still refused, tried other outlets, tried a different wire, checked everything in the computer, tried using the reset button as a power switch, and still nothing.
 
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