That's very unfortunate to have that happen.
But the engine possibly might not be seized. I'll explain.
Back up a few years - I bought a 2003 Duramax LB7 in a 3500 dually. It went through coolant for a short time, then it was fine for a couple years. Then the problem came back with a vengeance. It was losing coolant at an alarming rate.
In and out of the shop, still on GM's nickel. They have this really poor diagnosis method where the mechanic cannot use his own expertise and judgement but rather must follow a pre-determined flowchart style analysis to fix the problem. (No wonder GM nearly went under) The mechanic called GM tech support with the symptoms and tech support says do this this and this. If that doesn't cure it, do this and so on until they figure its time to put an engine in it. Sad procedure. Mine had injectors, injector sleeves, head gaskets and a bunch of other stuff.
Nothing cured the problem. After taking 38 quarts of coolant to make it from Vancouver to Calgary, I was so disgusted that I drove it into a dealer and traded it for the one I have now, taking a huge bath on the price of the truck.
I learned later that they discovered the problem was something internal in the engine that was missed from the factory, but I never learned just what the issue actually was. They wouldn't tell me.
OK, back to yours.
During the time I was driving with the coolant loss out the tailpipe problem, I would shut the thing off and come back later to drive it again. It wouldn't start - same symptom as you describe - it would sometimes just grunt, sometimes it would turn over and WHAM! it would stop dead from turning over.
The issue was a cylinder full of coolant, or partly full. This causes the engine to "hydraulic", meaning the piston comes up for compression but water does not compress, air does. With the coolant not compressing, the engine stops dead right now and will not turn over.
I am suggesting that maybe, just maybe yours has the same issue. The fact that you drove it in and shut it off and then it would not start again suggests that there is a possibility that you may have a cylinder full of coolant. It is worth checking out. If it is not seized, then perhaps most of the engine is fine and the problem itself could be fixed.
Either way, it is still bad news and I wish you the best in remedying the problem.
Please let us know what the outcome is.
When I traded my 2003, plagued with problems at 55,000 miles for the current D/A 2007 LBZ, I have never looked back. This truck, along with Dmax and Allison have been flawless in operation for nearly 7 years now. I have about 208,000K on it which is roughly 130,000 miles.
Don't lose hope - any make or model can have an issue. Buying a diesel truck was the best choice I ever made. Never mind these naysayers of diesel engines - they have no basis for fact.
My current ride has been fantastic all around, even after some drunk nearly totaled it.
