While your PC is running, take a look at the card itself. Is the green light flashing?
If it's not.. could be either that the card isn't being supplied enough power so you can try removing non-essential PCI cards (such as sound cards) to see if that helps.. Or try another PCI slot entirely.
You might also want to check to see if the card got dislodged somehow.. so also remove/re-insert the card and reboot windows. Once you're at your desktop, again, look for the flashing green light to at least know its getting power.
Don't feel bad, my card will sometimes randomly stop working as well, but a reboot usually fixes mine.