It wouldn’t be the first time a cable got stuck. In neutral, you ARE limited to lower RPM, so first, shift into forward and try the throttle. If it still doesn’t move, apply a moderate throttle twist, back and forth, while observing the cables closely. You should be able to notice which one is stuck, by seeing a slight movement, or lack there of. Maybe both of them? Anyway, from there, it is divide and conquer…you have to start taking things apart to see what is stuck, exactly. Sometimes it’s the tiller handle, sometimes its the trigger plate….
Long live American manufacturing!