|Is it a glass fuse:|
As already mentioned, your PSU will have a fuse--usually a small glass fuse like the above. If that''s where it goes there should be an empty fuse holder inside. Or, if the fuse has leads then it's meant to be soldered in
But it's unlikely the PSU fuse just 'fell out'.
Motherboards used to (and may still) have a small inline keyboard fuse that looks like a small resistor. If that blows your keyboard doesn't work. Other than that I don't recall any other fuse mounted on the motherboard.
Could there be anything else in the case that might be fused?
Maybe you've got some little kids running around the house? More than once I've worked on a computer that had something odd shoved in the floppy drive or other available openings.
Obama's a 2012'er. That explains EVERYTHING..