"Hydraulic lock , as uncle Bob says. It can be Common with any engine that coolant leaks into a cylinder. My old trusty F350 DRW 460 pulled the same trick. Number 6 cylinder leaked enough coolant after sitting on the jobsite for a 5-6 day period it wouldn't turn over. Hydraulic locked.
A local Shop pulled the heads and found no piston/cylinder damage done. Replaced the head gasket and ran the truck for another 25k before selling it.
I would advise talking with a "real" diesel mechanic that knows what he is doing and how to fix the issue and let them pull the heads and make a professional diagnosis as to what has happened and what the best route to get it fixed "RIGHT".
