I would begin by removing the starter (you might have to pull the oil filter to get it off). I have had water in my starter in the past cause it to rust; yours may be frozen. Check the starter while it is off by hooking it to the battery and make sure it turns. If not, get it warm and try again, or open it up and check it.
A 15/16 wrench fits the nut in front of the crank pulley on my tractor. You could also turn it by applying a screwdriver or small pry bar through the starter opening, but be careful not to mess up the teeth. If you can turn the crank either of these ways then it isn't stuck.
Good luck with your troubleshooting.