Hi Team,
as ever resist the urge to force the rack back and forth as you run the risk of moving the quadrant gear on the stuck plunger--that injection pump will then be out of calibration compared to the others--this then requires a re-calibration (Delivery volume) on a test bench or replacement pump unit.
Also depending on which way the quadrant gear was moved that pump will pump more or less fuel and if more could cause over speed to occur.

The usual method to unstick a plunger is to gently tap the plunger lifter up and down--usually does not take much force and the plunger that is stuck will be staying higher than the others and so not rising and falling by the pump's cam action so easy to spot with the pump side cover off and turning the main over.
You can gently apply a big screw driver blade under the lifter yoke and prise the lifter and hence the stuck plunger up once you have it moving to aid loosening the then possibly sticky plunger quicker.
Do not lever up or down on the plunger itself or its gear or you run the risk of breaking the plunger end off in the yoke and you will then need a replacement pump unit.
Cheers,
Eddie B.

When in doubt, some whip the hammer out! I wonder if anyone would allow me to try freeing up their rack with my trusty "SledgoMatic".JM

What can you say about your D4 but not about you wife. "Free up the rack with a hammer."

stake the inspection cover off and spray everything with penetrating oil or a mix of diesel with a little oil added to it and let it soak for a couple of days try and move it by hand each day giving it another squirt of penetrant. eventually it will free up.