what is "won't start"? you are going to have to explain exactly what isn't starting. are fans spinning up? I'm going to assume that you at the very least aren't getting POST when it won't start so I'm going to guess some kind of power supply issue, or some kind of hardware issue with the motherboard.
It could also be a temperature issue so you should install speedfan and see if anything is getting to hot. That's probably the first step. Using speedfan you can do a couple of things that you can't do in the Bios, you can test for temperature and voltage under load conditions, which can change the values quite a bit. It also makes it easier to keep an eye on it because it can show variables in a nice graph format and gives you some broad guidelines for temperatures using little icons. It's not a perfect measurement though (especially voltage) but is enough to indicate problems.
It won't start as in it when I turn the psu on the green light on the motherboard signals that power is running through the board(but the lights on the graphics card doesn't go on, which it's supposed to do) when I turn on my power, those lights on the graphics card are now on but nothing else is on. That means the fans cpu or anything else, my dvd burner won't even open. My mem ok button doesn't respond at all its like there is not enough juice in the system, but I bought a 750 W sea-sonic power supply. I hope this description was good enough