Had the same thing happen to me, but with a 6870. I dos-booted off a USB drive to see if atiflash found the device. Sure enough, it was picked up. So from there I did the standard things. Reseated the card(don't underestimate vibrations from all of the fans in the rig). Then I flashed a different, but slightly underclocked bios(this will void the warranty, and may not even work.) Then tried a different pci-e slot. And lastly, I just swapped power cables to work off different rails.
Well, all that fiddling, and it eventually came up. RMA first, if you still have that option. But if you flash the bios, be 100% sure that it's the right bios for the card, as revisions and reference cards can seem identical, when they aren't. Also understand that you're responsible for any/all damage.
I've already tried reseating and different PCIe slots, but I haven't tried atiflash yet. I'll do some reading on that and give it a shot, thanks for the suggestion!