I'm gonna ask a question, and just so you don't think I'm trying to be insulting, I'll tell a little story first.
I was doing some work in my new build a couple of weeks ago. I was putting in a bigger CPU fan. When I set it all back up, there was no boot. No power, no lights... I thought I had screwed something up for sure. I checked and checked everything inside the case, scratched my head, and was a bout to have a full on breakdown because I'd broken my brand new machine. Then I remembered that I had shut off the main power switch on the back of the power supply.
Your cousin didn't turn off that little switch by any chance?
And a broken anything can keep the PC from booting up. I just fried a RAM stick last weekend, and the machine just froze. When I tried to reboot, it was a no go. No status beeps or HDD activity. It just sat there dead until I shut it off and pulled out the bad stick, so you can't always count on the diagnostic beeps. But if the GPU fan isn't turning, I'd suspect a power issue first. Pull the card and see if the PC will boot without it, then put it back in.