if you can't move the shifter itself, it most likely is a linkage problem.
disconect it at the tranny and try moving the shifter, also try moving the part on the tranny. a long screwdrive should make it easy work, but you should be able to move it with your hand, it'll be a little tough thouhg.
if its the tranny, youll need to drop the pan and see what is up. theres not much to it. if it sat long it could be that water got in there and froze.