This happens from time to time in egg-laying species. For some reason they hatch prematurely prior to complete development. One of the theories I have read and believe to be true is that the underdeveloped snakes hatch due to the hormone trigger released when other snakes in the clutch begin hatching. As for what to do, there isn't much you can do. I'd recommend keeping the baby on a little more humid substrate than normal to prevent it drying out too soon. It may develop on its own and the snake may live, but I wouldn't count on it. Good luck.

Bryan
-----
"America was designed to answer to no one but ourselves, but somewhere along the line we stopped asking questions."
-Bryan Cole