Assuming its a model B Allis.
Need a little more information but going from what I understand.
The tractor was shifting o.k. and now the clutch is not disengaging.
My guess is that it was probably parked you came out later on and it won't disengage.
The way I solved this problem on my Sc was to put it in gear while it was off and start it in gear chained to a stump or solid object an give it a good pull with the clutch disengaged so it breaks (spins) it free. Keep a hand on the kill switch just in case it wanted to go over backwards. Worked like a charm. Now I park it with the clutch disengaged.
Good luck and be safe.