If a car overheats (for whatever reason, but a blown head gasket is a prime cause of this) the damage to the engine internally can be catastrophic. Pretty much any component can be compromised after an overheating event, so knowing where to start looking is the big question here.
Changing the head gasket requires removal of the cylinder head, and reassembly involves making sure that the camshaft timing is reinstated correctly. If there’s been a mistake made in this regard, the engine will almost certainly not run.
Certainly, injector failure is not unknown in modern turbo-diesels, but the fuelling system on a modern, common-rail turbo-diesel is a complex, fine-tolerance arrangement, so you also need to check the filters, fuel pump(s) and operating pressures. Even then, you might find that a simple, cheap-to-replace sensor is the single component preventing the vehicle from running.
I’d start with an electronic interrogation of the car’s computer. The problem there is that if the car hasn’t actually run with the issue that’s preventing it from starting, the computer may not have had the opportunity to log the problem in the first place. That said, a simple fault code might be all you need to know to move forward, so a scan is in order. Beyond that, it’s back to first principles, checking the timing and clearances of all the mechanical bits and pieces, including having the injectors bench-tested.