New Built Computer getting a black screen NEED HELP!

Hi,

I have just built a brand new computer. Everything is powered up and running: cooling system, fans, cd tray ect. When I first turned the computer on something came up on the screen and went away it was like a boot error. Now everything I turn it on I just get a black screen. I tired my old video card that works on my old computer and got the same result. My motherboard is a Z68XP-UD3. My video card is a Sapphire Radeon HD 6870. My processor is a Intel Core i5. I have 8GB Corsair Vengeance Ram (4 Slots 2GB a piece). Finally my power supply is GS700 Corsair Gaming Series (700 Watts). I'd appricate some help with this cause I really don't know what to do.

Thanks!