Sorry for your loss. It is common to lose a chicken once in awhile. Sometimes they may have an ailment that isn't apparent unless you spend a lot of time with them. Some can go about their daily activites and just drop dead. Heart failure can happen, and liver or kidney disease are sometimes problems. There are several reproductive diseases that are very common in chickens, including internal laying, egg yolk peritonitis, and cancer. They can become egg bound, they may spend a lot of time in a nest box with that. Does she appear to have lost weight in her breast area? Can you feel inside her vent for a stuck egg. Is her crop full and hard or like a balloon? Did you notice any evidence of lice or mites? Had she been wormed? Was she eating and drinking? Did you notice her droppings? These are some of the things to look for in a sick chicken. Hot temperatures in coops or chicken yards this time of year can sometimes cause heat stroke, especially if they are not drinking enough or being kept from water by bullying. If you refrigerate her body, you can get a necropsy done in Texas in one of several places, or by contacting your state vet or local extension agent.