The most common reason for me for every machine I've ever owned that just won't turn on at all is a failed power supply. Those are the big noisy box things in the case where the power cord goes in, and all the little wires come out. They are cheap to replace (a new one can be as little as 20 dollars) and easy, requiring only a screw driver. Just mark your cables leading to each part of your motherboard and drive before you disconnect the bad one, unscrew it, pop out the old one, slide in the new one, reconnect the cables, and you're off.