Welcome to the Ford/New Holland Forum.
My understanding of the original note is that he shifted the HI-LO shifter to HI and it is stuck in HI.
Put your shift levers in neutral and remove the transmission cover. Take care not to lose anything into the transmission. There may be some spring loaded detents held down by the cover. Just raise the cover carefully and they should stay in place.
Disconnect the safety switch wires. and put the cover aside.
Study the shift forks and you will see that the HI-LO fork is not in neutral. Take a big screwdriver and move it back to neutral.
Button her up and see how it works.