I lost my first hen the same way just a few months ago. She was a 2 year old RIR who became lethargic. As soon as she became lethargic her crop filled up. It was so full it would leak from her beak. She died 3 days after exhibiting symptoms, and it turned out (upon necropsy) that she was internally laying. Because of the advanced stage it was in, her digestive tract slowed down contributing to the crop issues.She died on my lap. I don't Think I held Her upside down long enough and I think She aspirated. By older...I meant 2 years old. She was part of our first chickens we got. We all loved her so much. She was perfectly fine the day before and she was leaking from her beak and her vent.
If your hen was a very productive egg-laying breed, it is more likely that she had a reproductive problem than a hen that was not so productive. I don't know what breed your hen was or if she exhibited any other symptoms though, so it could have been something else of course. But often (especially with older hens, and not always of course), problems with the crop can indicate more serious underlying health issues.