How about the seat safety switch which will not allow a pto to be engaged on a tractor with out an operator on the seat or with the bypass switch engaged. You said the tractor stopped, not locked up. If the pto locked up there would be some serious noise involved I would think. On my old Deere you can get off the seat, flip it forward and pull up the switch to run a logging winch or post hole digger etc. As soon as you set on the seat it pushes the mechanical switch down and resets the safety feature. My guess it's simple and just a wiring issue or circuit fuse issue. For what it's worth...
