Well, either the engine has to be running or the wheels have to be turning to change gears and that's why I asked.
OK - here's a kinda off the wall possibility: I knew one dumbass who removed the chain guide (affectionately called a case saver) from under the sprocket cover and put the bolts back into the holes - one of the bolts bottomed out in the hole and broke through into the transmission obstructing the shift mechanism from moving out of neutral - low and neutral was all he had - when he started tearing down the motor to fix the transmission, he found after he removed the chain guide mounting bolts, the transmission would magically shift - he figured out what he had done when he filled the engine with oil and it ran out of the bolt hole by the sprocket - he put the bolt back in to stop the oil and found out it would not shift again - he had to get a shorter bolt to put in the hole and stop the oil while not obstructing the shifter.
Maybe? Check it out.