earth
i have had that problem so many times. all of those times it was because the motherboard was earthing. something about the mobo touching a part of the case is shouldnt. but to make sure the mobo hasnt blown take it out the case altogether, sit it somewher (say on a pillow) , put power in2 it an get a screwdriver, an uno the little bit wher u plug the power cable switch, along with teh hdd light, reset switch (i dont know the name of this bit,jus aloada pins lol) put a screwdriver in there, just so its touching the bits of metal and teh mobo should start up. if it doesnt the mobo might have blown. if it does the mobos fine. you just gotta be careful when your puting it in. try and test it everytime you put a screw in, and use risers if you can. everytime i have had this problem i took out the motherboard an put it back in and it was fine.
p.s. it dont matter when or how it happens, you dont have 2 hit it. what im saying is you could be sitting and for no reason it powers off and wont start back u, liek what happened to me. try what i said above and you should be fine :grinthumb