It's probably been setting for a while, hasn't it?
Anyway...sometimes you can break a frozen clutch by aggressively starting and accelerating several times. You might try forward and reversing too, if you can switch gears.
If that doesn't do it, you probably have to split the tractor and replace the clutch, pressure plate and throw out bearing.
Sorry for the bad news...but that's normally the drill with rust welded clutches.
If you do manage to break it free, make sure you block the clutch (use a piece of wood or something on the clutch pedal) during periods of extended inactivity..or exercise it once a week (set on the seat and pump the clutch pedal a dozen times or so).
Good luck!