Maybe the compression you felt was old dried up oil from fuel, did it use to be really free feeling with a pop when u turned it over? Obviously a lot of compression in the pop... or was it rigid and firm turning the flywheel with no pop?

If it indeed went from having a smooth rotation with a compression pop to having absolutely nothing then the only thing I can think is your glow plug isn’t in it, or the very top of the block has cracked (which would be the first time that’s ever happened that I know) there’s a good chance somethings not right or your engine is really old. If u have the time just tear down the engine, they are very basic to understand u do not have to put them together with timing measurements or anything, I could tear a new engine down and back together from running to torn down and back running in probably 25 minutes and 3-4 tools.