Easy thing to do here - take the one card out to see if the other still works.
Easiest to take the top one out 1st as you can still run the bottom one with SLI switched on.
If that works great, then put the other card in the bottom one, if that works great because what it means is that your windows driver is corrupt - time to run rescue disk or re-install.
If you been playing with the BIOS then you need to reload the original also.
I found that if you take the cards out and put them back again - that sometimes helps clear the issues. Even if the card appears borked, it might not be.
Hope it's not bad news.
Cheers
Mav