Try putting the tractor in the HIGHEST gear, then with the engine shut down, pull the tractor alternately a few feet forward then a few feet backward with the clutch pedal fully depressed. The torque reversals will usually break it free after a few cycles.
If that doesn't work, get the tractor running at idle and "float shift" it thru the gears until it's in it's highest gear. Then open the throttle, depress the clutch fully and try to stall it using the brakes. The clutch should break free suddenly - be prepared!
Important: Do this where you have plenty of room, can run the tractor safely in road gear and there is NO traffic!