I had something similar with one of mine a while ago. It go stuck in first though. It turned out that the selector at the base of the gear stick had jumped out.
To fix mine, I lifted out the gear stick by undoing the three(?) torqs bolts at the base, lined up all the sliders underneath where the stick was bolted in so that the box is in neutral and then droppped the stick back in. It doesn't look possible but trust me, the selector can slip out. May not be what has happened to yours but its a fairly easy thing to try.....
If its not that then it may be something more serious such as mechanical failure of the selector forks inside the box.
Fingers crossed for you.
Transit...... The first and best MPV