I normally leave my computer on, but one day I came home and noticed it was off. I went to turn it back on and nothing happens at all. No sounds, no nothing. The power light in the back off the desktop is on though. So I have zero idea what could be wrong with it.

There are two possibilities here:
The computer's PSU gave out - it is no more

A power glitch zapped something.

If you're lucky, it's a fuse (if the PSU has one); if you're less lucky, a new PSU might do it, but there could be collateral damage on your mobo or RAM.

If the fans don't come on and the disk doesn't spin then unless the power switch is the problem (why should it be), I suspect the PSYU.

try disconnecting the psu from the pc line for about 3 mins. then put it back and power it on, check if it will work. if it doesn't then try what Suspishio suggested..

Do you have a surge protector??
Probably an electric surge fried up your PSU.
It is a cheap easy replacement, google power supply. 400 watts should be enough. And get your self a surge protector if your gonna leave ur pc on constantly.
Hope it did't get to your mother board