I am having the same problem. 2 X EVGA GTX260 were working fine till last night. Power this morning, and one card disappeared. The 3 monitors power up, but then it says no signal. so now only card is seen in device manager. Also, the resolution went down. 2 of the 3 monitors are in stanby, so I am pretty sure the graphic card is fine. PSU is 1000 watt , new PC HPVoodooDNA. I think it's something with windows 7.
What I would do to confirm both cards are good is to try each alone (as in put in the primary slot and physically remove the other). You could also try reseating the cards. I had a similar thing happen a few years ago to my SLI rig, and it turned out a motherboard component had gone bad, but it did not affect the PC otherwise. What type of PC is it?