computer failed to turn on last night, then...
 

It magically started working today. My gf was online, and she says the comp just shut down. She checked all the connections, and the power switch of course. Nothing. I woke up this morning thinking my comp died, but I tried the power button again, and lo and behold, it booted up. All my other connections on my powerbar working fine.

What could cause this?

Could be a million things, but seeing that it turned itself off while in use, and then would only turn on again after a long wait, I'd check if the cpu fan is working properly.

Most motherboards monitor heat buildup and/or fan speed and will shutdown automatically to prevent damage. You try to turn it on and it wont because its still too hot, but leave it a while and then it works again.

I'd check the PSU as well. I've had a problem in the past, with a cheap power supply, where a capacitor would blow, and it would start to overheat, shutting itself down once the temps get too high. Symptoms included: a burning odor from inside the PSU, and a warmer-than-usual surface. The problem was only fixed by replacing the PSU completely.

Definitely check your CPU temps as well, like tekaweni wrote.

I would bet its a power supply going out

My bet is the PSU crapping out. It could also be an overheating issue, but my money is on the PSU.