Sorry for your loss. If you would like to find a cause of death you could open her up to see if she was egg bound, or had anything out of the ordinary. Or you could refrigerate her body, and ship or take it to your state vet for a necropsy. Cost varies with each state, from free to $200. Ohio is $80 where I live. At the point of laying, occasionally a pullet will die to one of many different problems, such egg binding, heart, kidney or liver failure. Hopefully, your other chicken may just be getting ready to start laying. If you have a regular vet, I would take in a few fresh droppings for a fecal test for worms and coccidiosis. I would put on a rubber glove, and insert a finger into her vent 1 -2 inches to check for an egg which should feel hard. If it is very hot where you live, make sure they are getting plenty of cool shaded water to drink.