Sometimes you can unstick them without even cranking the engine. Put it in high gear, then rock it back and forth using the rear tire to push on. The back and forth action will cause it to break free if not stuck bad. Other times I have seen folks put them in high gear or next to high gear and pull the tractor with another tractor, truck or what ever you have to pull it and just skid it around till it breaks free. You can alternate between high gear and reverse to make the clutch turn back and forth if the tires continue to skid. This sometimes helps to break a really stuck one.
In the future (after you get it unstuck), always fully depress the clutch put a block of wood between the clutch pedal and the tractor frame or step to keep it fully depressed. Keep this block where you park your tractor and use it every time.
Some tractors are really bad to stick the clutch disk while others never stick.