Yes, this is tragic, and I do understand as it's happened to me. Coco was a stunning hen, gorgeous, and I know she'll be missed.
Over many years of keeping chickens, I've had many die of many different things, but most of them were not sudden and without warning like Coco's death. The ones that did die suddenly, I sadly traced to insecticide poisoning or other petroleum distillates such as pain thinner contaminated soil and hydraulic fluid contaminated soil.
Even a very voracious bacterial infection that kills in less than 24 hours will cause a chicken to be visibly sick. Hens can die from reproductive disorders, but those also make a chicken sick before she dies.
Some folks here will say that a chicken can die suddenly of heart failure. I had a young rooster turn up dead under his roost one morning. He, like your Coco, had been perfectly fine when I closed the coop the night before. I suppose it could have been his heart. But more than likely, he had tumors on his heart or other organs and they just quit working. I've had it confirmed I have a cancer causing virus in my flock.
The only way to know for sure what killed sweet Coco is to have her body necropsied, opened up and organs examined and fluids tested. I'm glad you have already made arrangements. Please let us know the results here on this thread.