probably a stupid question but..... did you remember to plug the power supply into it? I had an idiot moment that lasted an hour or so accompanied by sheer panic a few months ago when i was messing around in my case. I forgot to plug the PSU connectors back into the card, I thought I'd ruined my card..... thankfully it's still healthy and mining full swing atm with it's lil bro 5770
Otherwise I'd guess the card's bad if a dummy plug didn't work. Hope you get it resolved