Hp pavilion motherboard troubles.

Hi! I'm new to self computer repair (trying to save money) and I need help.
I have a Hp Pavilion a430n model that will not turn on. It went through two power outadges in one hour and I think it fried the motherboard. I spent two hours running diagnostics on the darn thing. The power source works (little green light stays lit when it's not plugged into the motherboard), the harddrive works, the disk drives, cd-rom, dvd-rom, all work. But when I plug the power source up to the motherboard the little green light on the back of the power source flashes and nothing turns on. Do I need to replace the motherboard or try somethin else?
And if I go to replace the motherboard, where should I try to find one? (Also trying to upgrade the processor, ram, and graphics card at the same time. My husband wants to be able to plays games on this one and it's not powerful enough.)