TheFragileI'm assuming you don't have a motherboard with LED codes, because that would be helpful to see its thought process upon turning it on.
It sounds like it could be a motherboard issue, especially if you can't tell if it does anything at all upon turning it on. My roommate had a drink spilled on his computer, and his motherboard got ruined. Like your scenario, there would be no display and we couldn't tell if it got anywhere at all upon turning on. I would entertain the idea of taking out your video card and trying to use onboard video, just to make sure it's not some weird video card issue (honestly unlikely but you might as well try). Other than that, it sounds like it's the motherboard, which sucks. :(
Funny thing, it was raining very lightly earlier today and the window behind my PC was slightly open...