Back to the Fuel Injection Pump itself.

If the Elements (Plungers and Barrels) were starting to get worn out what would happen is it would start OK cold but would have trouble starting when it was hot.

If there was any Water in the Fuel it is possible for Rust to develop in the Element or Plunger or Barrels and what can happens is the Plunger goes up and jams in place and the Spring cannot return it.

You can do this only on an MW type Fuel Injection Pump. Similar to what you would do if you were Drip Timing. You remove the Delivery Valve Holder the Srping, the Centeral Valve and you will add removing the Barrel that the central Valve went into and the Crush Washer.

You can now look down inside and see the Plunger down in the Barrel of the Element. You can watch that as someone rotates the Engine by Hand for you and you can see if the Plunger is moving up and down as it is suppsed to.

If it is not moving remove the Fuel Supply/Lift Pump (about a Pint of Oil is going to come out) and have someone hand crank the Engine and see if you can see the Camshaft Moving. If the IP Camshaft is moving but the Plunger is not your Element Plunger is stuck.
If the IP Camshaft is not moving that is another issue.
It is not unknow for the Camshaft to break but it seldom Happens. I have only seen that happen 2 times in 5 years working in Fuel Injection.
More common is the Drive end Nut or Bolt coming loose and the drive end shears off the Woodruff Key and spins.

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84 300D, 82 Volvo 244Gl Diesel