You probably bought a bad computer. If I were you, I'd take the graphics card out, and take it to ASUS. Don't tell them you added a graphics card on your own, they could use it as reason (though stupid one, no PC should just stop working from graphics card) not to accept the return. Just tell them that when you got it home, it didn't work from first boot.
Also, what are the specs and how much you paid for it? Pre built PCs tend to have terrible performance/price ratio, so if it's an option, it might be better getting a refund for it.