Put a small piece of wire through the hole in the nut and then screw it bask on. Temporary fix, but it usually works.
Another idea is to use a small amount of 0000 steel wool in the hole with a toothpick and then reuse the original screw. I actually saw a dash screw attached using a wing nut. To access the wig nut you swing out the windshield. Kinda "screwy" but effective.