I've had a similar problem on one of my machines. I put the box blade rippers down, then dropped the blade into the dirt, and pressed in on the clutch most of the way.
That let the clutch slip a little bit, and free the corroded material that was bonding the clutch plate onto the disk. I also lubricated the clutch release bearing, and adjusted the free play in the pedal first. A couple cycles of slipping the clutch freed everything up, and now it works fine. Good luck!