Originally Posted by dkerwood
The nut the left side of the hinge has gone missing, allowing the pin to slide out.
Take out the pin on the right side, go to any store that sells screws and get something with a thread and length that's about the same dimensions. The head style doesn't really matter if this is something you're not going to be adjusting too much.
Of course, best option is to get a direct replacement from the company. At the same time, a screw is a screw is a screw (!)