I don't know your tractor but when my Mitsubishi would stick in second I could remove the plate holding the gearshift to expose the gears inside. I could then use a pry bar or screwdriver to pop it out of gear. I could then put it all together again and it would work fine until the next time.
When you remove the top plate, check to see if the shifter will move. Sometimes there is a spring detent ball that is stuck. Or check your gears to see if they are stuck, and why.
This happened to my Ford 1720 a couple of times. Was able to rock the tractor with the front end loader enough to take the pressure off the transmission and pop it into gear. I'm careful now when shifting gears and it hasn't happened since. 1450hrs are on the hour meter.
When you remove the top plate, check to see if the shifter will move. Sometimes there is a spring detent ball that is stuck. Or check your gears to see if they are stuck, and why.
Yep. That's usually where that problem is. Often an easy fix and sometimes due to rainwater running down the shifter. Take the cover off and have a look.