Welcome to TBN. Sounds like a stuck rack in your injection pump. Don't start it until mare knowledgeable people about your specific model tractor chime in here.
Also state how many hours on the tractor, etc.
G'day check the throttle linkage on the injector pump should be on the L/H side of the engine at the front, check to make sure the throttle lever ( the rear one from memory ) is free and able to move through all its range it should sit back against a small bolt with a 5/16 head on it in the middle of the top cover. If it is then it is prob a govenor failure and that will require removal of injector pump and getting it rebuilt.
I know nothing about this machine, but I have had a similar problem that turned out to be a plugged return line from the injector pump. It's worth a look.