Two possibilities here: first, the pilot bearing in the flywheel could be locked up and driving on the stub of the input shaft. Not very common, but I have seen it before.
Second and more likely, is that the clutch plate has rusted itself to the flywheel or the pressure plate. This usually happens if a tractor sits for any somewhat lengthy period of time. If it hasnt been that long, it may be able to be freed up by putting the tractor in HIGH GEAR (this gives the most leverage at the tires) and rocking the tractor with the engine off. Make sure the clutch pedal is wired or bungee strapped down, and if the plate is lightly stuck it may break free.
Sadly theres not a great way to tell which problem it is without splitting the tractor.
Hope this helps