What type of cards are you running?
Have you tried your cards one at a time not SLI/crossfire to see if just one of your cards are bad?
If neither worked, did you try to use your motherboards onboard GPU to see if it's you MOBO that's bad and not your cards? (hell MOBOs are cheaper than vid cards most of the time)
If only one of your cards are bad you could just run one card or buy another of the same to still run them "linked"