Art is right, JCP, your pressure plate, clutch disk, and flywheel are stuck
together. I had one of those L2500s with crashbox-type trrans. Try
this: start the tractor in low gear. Drive forward. Press the clutch pedal.
Hit the brakes hard. This may or may not work. You must go faster than
idle speed or you will stall the engine. Be careful.
One other possibility is your clutch linkage is messed up enough not to
disengage the clutch.