This is not an uncommon problem. That's why many manuals recommend locking the clutch in a depressed position when parking the tractor for storage. Clint's suggestion should work if not totally frozen. Trying in a higher gear may work better than in a lower gear.
I've heard starting engine & operating engine for sufficient time to attain operating temp with tractor clutch pedal depressed could possibly free clutch. If clutch housing has inspection cover I'll suggest to spray clutch/pressure plate with brake cleaner. Ditto on disengaging clutch if tractor is to be parked for any length of time.