This problem is not uncommon. There have been posts in the past in which some tractors have a hole in the housing in which you can insert a screwdriver and pry the clutch loose. My Kubota B2710 came with a wooden block to use to hold the clutch pedal down when not in use to prevent the clutch sticking, although I never used it and the clutch never stuck, but the tractor was always kept inside in my shop when not in use.
My first experience with a sticking clutch was a 1940 John Deere Model L when I was about 12 years old. And I learned to do as Wnc3 says above. We kept that tractor parked behind the barn and had a little open space in front of it. So I'd put the transmission in neutral and start it (hand crank in the front end) and get it a little bit warmed up so it would start easily the next time. Then I'd kill the engine, put the transmission in the lowest gear (3 speed transmission), set the throttle to idle, pull that crank to start the engine, get out of the way to let it go by, then climb on from the back end, push the clutch pedal all the way down, open the throttle wide open, and stand on the brake pedal as hard as a 12 year old could.:laughing: That always worked, and it wouldn't stick again until the tractor had been left unused for a week or more.