Sounds like the rack may be siezed in the pump. You can take the side plate off the side of the pump, and see if the pump elements are moving when you move the governor linkage to high idle. If nothing moves, the rack is stuck. You can't see the rack it is behind the pump elements.
You can spray all the internals with a penetrating fluid, and force it with a screw driver or something to get it moving the elements rotate anti-clockwise from the top where the little screws are that clamp the pump elements), fill the pump housing and governor housing up with diesel, and run it. But be very mindful that if you start it before you get it free enough, the engine may run away, and you'll end up with bits of engine on the ground, and possibly in you

So if you do decide to free it up yourself, take the air intake hose off the inlet manifold, and have something solid ready to block it off if things go bad, quickly!!
Once you have it all freed up, and flush the housings a few times with diesel, don't forget to drain them both, and refill with clean engine oil.
Once, again, DO NOT start it until you have the rack moving nice and freely! Spray everything you can with a good penetrant, and let it sit a while, and repeat a few times before you try forcing anything.
Good luck!