Looks like I'm a bit late...
Anyway, from what I gather, the comp isn't posting BIOS and thus, isn't starting up at all (no video feed, no beeps on start up, ect). If it doesn't post bios I would point the finger at the mobo/PSU. I think a computer doesn't need anything except RAM and a mobo to post BIOS. Correct me if I'm wrong here.
If it does post bios but doesn't get past it (to loading the OS), could be any number of things from failed HD, RAM incompatabilities (unlikely since nothing has been changed), RAM failure (very likely). IRQ conflicts can cause wierd shit to happen as well.
While I realize the above doesn't help much... I do just want to point out that I think it may be random coincidence that it up and died when your roommate was using it. It certainly doesn't sound like a software issue here. Sounds like something is baked, bettered, and fried. Software problems tend to happen in the operating system environment... which doesn't seem to be the case here.
Good luck.