Do that in a low gear, idling - you don't want to pop yourself over the hood.... !!!!!
Most likely your clutch rusted/ froze onto your flywheel.
Have you put a block of wood under the clutch pedal to keep the clutch pushed in for a few days, see if it would drop free on it's own? I had this problem once, and this worked for me.
There are a dozen other things that it 'could' be. But best bet is a frozen up clutch, which you need to let fall loose (with the block) or shock loose by driving it with the clutch pushed in and adding more pull to it.
I had one tractor the clutch wouldn't disengage. The shaft that went in to the clutch, the welded on arm came loose, the weld was bad, and at 3000 hours on the tractor this arm slowly rotated on the clutch shaft. About drove me nuts trying to figure out what was going on, as it still grabbed the shaft pretty good, was very hard to figure out....
--->Paul