Ok, so I had two HD5770's running crossfire and all of a sudden on PCI slot just stopped working. I have tried updating bios, updating driver, checking bios, re-installing chip set driver, checked the voltage going to card, and the PCI slot is getting power.
Have you tried swapping cards to be sure it's the slot? Swapping power cables to the cards? Try each card alone?
I have tried swapping power cables to the cards. I've also tried swapping the cards into opposite slots in other words putting the top card in the bottom slot and the bottom card in the top slot. The bottom PCI slot is the one giving me problems.
Both slots recieve their +12V power from the same 24 pin motherboard power cable, so it's unlikely the PSU is at fault. However, that being said...
The 24 pin power cable has 2 +12V yellow wires; pins 10 and 11. You might want to check and see if they both read +12V to ground (pin 24) just in case one of the circuits went dead and the PCIe x16 slots each use a different yellow wire to power thier +12V circuit. http://pinouts.ru/Power/atx_v2_pinout.shtml
If all is well there, I would think the slot itself failed or the chipset controling it failed.