as Bob said to get the gears unlocked:
over the years I have used the following procedure when shifting to keep the gears in my old TO-35 from locking up.
I always keep some downward pressure on the main shifter lever when shifting, you have to train yourself to do this,
the most difficult time is when you are backing up, stop, then shift to a forward gear, if you shift too fast as you apply the brakes your body will go backwards as you have the shifter in your hand and you will pull up on it as you shift.
then its hung.
learn to keep a slight downward pressure on the shifter lever as you shift!!!!!!!!!!!!!
always keep a wrench in the tractor to take off the trans oil fill plug and a long heavy screwdriver to reach into the gear case and unlock the gears.
good luck and yes you can learn new methods to shift after you get the transmission unstuck.
my old 1957 has not stuck in gear for many years now. but I always take my time shifting gears.