I read in the Reef Aquarium Vol. 1 that you should realign the shells and put a loose rubberband around the shells. The rubberband needs to be loose enough to allow the clam to open. It was said that the clam can secrete a new hinge in about 2 weeks.

In reality, I've never seen or heard of this working, but Julian Sprung's not a complete moron. So I guess it's worked before.