Happens a lot in old farm equipment. There can be 2 reasons: Clutch plate rusted to the pressure plate or clutch material deteriorating and swelling up.
The first can be broken loose by starting the engine in gear and driving it around with the clutch in until it brakes free. Chose the Highest gear you can start it in (2nd gear slightly down hill)then on and off the gas to brake it loose. Also can be done by towing or pushing it, clutch in, with another truck until it brakes loose.
If it is the second reason, nothing to do but take it apart and replace everything.
