PSU Problem?

[Triple-X]

I had my computer turned on in the morning, and I left it on and went to another room. After I came back, about 1 hour, it was off. I try to turn it on without any response. Then I reset everything and it turns on for about half a second and turns instantly off again. So I take out the PSU without anything disconnected, just unscrewed the PSU from case, and set it sideways next to my PC. Everything boots up fine. So what could be the issue? Out of case and sideways it works fine, and in the case the way it's supposed to be gives me the problem stated above. What's the problem?

 

[SYL\X/3K]

check if any pins are burned on the power plug that connects to the mobo, if so take a pair of pliers and bend the burned pin(s) slightly
the current of electricity isn't flowing properly and this is ur result

 

[Deathawk]

Maybe it was a one time thing.. try plugging everything in again, and putting it in the case, to see if it works...?