I think Charlie and Kirk gave you the best two possibilities to check out. If it was previously shifting well, and you can't find the pin that Charlie mentioned, there is a good possibility that it fell into the gear case. If the pin is in the gear case, and left there, it could totally destroy most of the gears in your box. To get it out, you could remove the bottom plug and drain the oil. If you are lucky, it will come out with the oil. If not you could go fishing for it with a small high strength magnet attached to a telescoping rod.