May have a hydraulic lock.
Coolant may have leaked into one cylinder from, as you say, a busted
headgasket or cracked head. If the engine turned at all, this cylinder
was soon in its compression stroke and, as we know, liquids don't
compress so the engine's rotation stopped. Or, if the engine didn't turn
at all, then one or another piston is seized and the whole thing must be
In the happier event of a hydraulic lock, removal of the glow plugs will
allow the trapped coolant to escape when the engine is cranked, or, less
happy, confirm that the engine is, in fact, seized.