Computer wont turn on

Earlier today i had gone out with some friends and my desktop was working fine when i left, i come back and it is on, but off at the same time(the green powerbutton lite was on, but the computer was off) so i held the power button down until it went off and tried turning it on, as this had happened a coupel times before, except this time, it didnt turn back on, the powerbutton lit up, but nothing. i know there is still power to the machine, as the green lite inside is on, and the powerbutton lites up, but what could be wrong with it? why wont my desktop turn on =(

Check the power connections in your computer. It could be that the processor (the CPU has its own power connector, independent of the main motherboard connector), hard drive, or some other component had its power connection come loose. Try swapping connectors if you have extras. If that doesn't work, start disconnecting components, until you just have the motherboard, CPU, and fans hooked up. If it boots, one of the perherprials is the problem. If none of that works, your power supply is probably FUBAR.

Oh, also try different outlets, to make sure it's not a problem  with the building's wiring.