Stuck clutch. You need access to the back side of the flywheel and the 6 bolts holding the pressure plate to the flywheel, either the inspection plate or starter.
Remove one of the bolts holding the pressure plate to the flywheel. Purchase 6 more bolts of the same diameter and thread count only about 1 inch longer. And 6 nuts one size larger than the bolt. You are going to use the nuts as spacers.
Remove the sparkplugs - you need to rotate the engine/clutch/pressure plate to do the following.
Install the longer bolt, snug up. Replace the remaining 5 bolts. Back out all 6 bolts about 1/2 inch to 3/4 inch, just a little bit at a time. The cover plate should be lifting off the flywheel. You may have to readjust (usually shorten) the clutch pedal linkage to operate the throw out bearing. Depress the clutch pedal (wife) - operate the clutch. At this point take a small pry bar and lift the edge of the pressure plate in 3 or 4 places.
Put the spacer (nut) between the cover plate and pressure plate with the bolt through the hole in the nut on all 6 bolts. Tighten up all six bolts.
Start the tractor in gear. Step on the clutch and brakes. If the disk is going to break loose this should do it.
Disclaimer. I have not tried this method of freeing up a clutch and make no guarantees. I take no responsibility for any damage or injuries. You are on your own.
Ok, now you can tell me I'm full of it. It ain't gonna work no way. I have never heard of anyone trying this, you are nuts.
Well, I have had to disassemble a clutch inside the tractor because the transmission input shaft was frozen to the pilot bearing - tractor wouldn't split.