My Pc will not tern on?

Ok I have just had a new graphics card and mother bored put into my pc .It has worked fine for about 2 weeks .Then it started ramdily terning itself of like if you had palled the power out. but it wood tern back on if you press the button.It did it today and woodnt tern on and still won't. the specks are 300g hard drive, pskpl-am epu mother bored ,nvidia geforce 8800 gtx, 2.5g ram. the sb pwr green LED on the mother bored is on so there is power going to the mother bored. but don't no what is wrong with it please help .ask me if you need more info. I'm sorry if some of my grammar is wrong I can't spell to save my life .