More information required. Are you saying the clutch disc won't release when the pedal is pressed in and thus you can't get it in gear once it's started?
Vic's method that I haven't tried but agree with in principle is thus: Start the motor in gear and drive it out to hard surface. Be careful, the only way to stop is to slap in in neutral when the transmission is unloaded (by holding the throttle just so or catching it at the right moment as you take your foot off the throttle) or to pull the compression release. Asphalt or concrete would be best but unless your road is lightly traveled that may be difficult. Hard flat dirt is preferable to gravel. Once you get there shut it down and make sure your brake pedals are locked together. Now restart in a one of the road gears, 7-8mph at max rpm is preferable. Run it up to full rpm, set the governor arm, then put the clutch in and stomp the brakes. Take care not to lock them, fish tailing something with a high center of gravity is nothing to laugh about. You may stall the engine once or twice but that should brake it loose. Slip the clutch a bit to clean the rust off but don't overheat it.