How do you get your Massey Ferguson to shift out of reverse it is stuck in reverse?

If the trans is locked in one gear, and the stick refuses to move, it has very likely jumped out of the shift selector rods. You will likely have to remove the shift stick cross pin, to pull the stick out, and manually with a long flat screw driver, shift all rods to the neutral position. There are cut aways in the rods. The neutral position should be when they are all in the same general location so the stick can move between them. Test this by bumping the starter, before replacing the stick.