Computer stopped turning on, not sure where to go from here, tried replacing PSU and Motherboard, no dice.

Hello! I'm not really a hardware guy but I am tech savvy.

My roommate has an older computer that he had built - and the case was warped, which eventually led to the power button failing. I showed him how to connect the two pins with a screwdriver to get the computer to boot while we figured out a replacement button, no sweat.

Then he says he stopped turning on - sure enough, when I went to try and turn it on the fans only rocked a little. I switched off and unplugged the PSU and plugged it back in, and then the fans wouldn't even rock. He said the last time he turned it on he had used the closest metal thing he had - a plugged in USB cable, and he's worried he shorted it out. Since the fans were rocking, I thought that was unlikely.

My initial thought was the power supply, so I picked up a new one, tried to install it - still nothing. This leads me to believe it was the motherboard, so we got a replacement one of those as well, but I just hooked it all up and I still can't get it to turn on.

Am I overlooking something?