okkkkey, we have lots of marine experts on duty here. So many solutions!
So few real details.
It wouldn't start after prolonged cranking,,,,, but was it cranking fast enough? And was it now too cold to start without glow plugs,,,, that weren't needed and haven't been working all summer? Who knows?!!!!
But I love a mystery! Don't you? The answer may help one of us in the future.
The bad battery may be the answer but I doubt it. How long has it been since it last run? Did it crank normaly or slow? Did it try to start?