My old Ferguson sticks in 2nd gear fairly often. In this type of tranny (gears moved directly by shift lever, without linkage) it is not too unusual for them to stick in one gear or another, and usually happens if someone forces the lever too hard of too fast. The lower end of the shift lever falls out of the slot it belongs in. On the Ferguson , I have a couple of ways of getting it out of gear, both pretty easy. The first most common way; There is a pin that holds the shift lever in place right where the lever goes into the tranny. I drive that pin out and then lift and re-insert the shift lever in a different place until it shifts. The second way is I pull the oil fill plug and physically slide the gears on their shaft with a long screwdriver. Both work. I have seen this done on other tractors too. Hopefully yours is similar.
I would pull the shift lever out and inspect it. There is a probably a rounded section on the bottom of the lever that fits into the shift mechanism. If that isn't broken off, try the above methods if they apply.