as others have said remove the large transmission oil fill cap
use a long screwdriver and try to move the slides on the shifter rods.
do you have plenty of oil in the transmission.
when was the last time you changed the transmission/hyd oil?
my old to35 used to lock up sometimes, but I learned to never, never pull up on the shift lever when I was shifting, that seemed to have some cause for my trans to lockup. so I trained my self to always place some down pressure on the shift lever when shifting.
did I say never pull up on the shift lever when shifting.
many times it will lock when you are going in reverse and reach to shift before you stop. with shifter in hand and your body goes back as you stop, you will pull up on the shift lever,
hey it just happens.
you can learn new tricks.
after you get it unstuck, just try not to ever lift up on the shift lever when you change gears.
good luck and let us know how it goes.
but do not beat and bang on the gears, you may have to call in a professional, if you can not get it unstuck.