Rock it back and forth to get it into gear. Easiest way I ever got something (old land rover) into gear was just start engine and start drifting the truck and then slide it into a gear as it's moving. Worked every time.
If brakes work, start the tractor in gear and while holding clutch in, push hard on the brakes. That usually will free it up. Make sure you have plenty of room so you don’t run over anything.
Stout tree method is best. Tractor brakes are iffy. Next time when you park it, put a block of wood between the pedal and the mount. Many tractors come with a pedal hook to keep the clutch depressed when parked so the plate don't rust to the flywheel.