So very sorry to hear this. It would be nearly impossible to diagnose what happened with her without knowing more, and despite this we'd be guessing. Can you consider getting her in for a necropsy? It will help you understand what happened, and possibly help the rest of your flock. Some states do it for free, or for a minimal fee. Where do you live? If you want to do a necropsy, put her in the refrigerator (NOT the freezer!!) and call around as soon as possible. It'd ideal to get her in within a day or two if you can.
Some avian vets can also preform a gross necropsy for you.
If you are unable to afford a necropsy, you can attempt to do one yourself. Are you familiar with chicken anatomy? There are some threads here that can help you if you search 'necropsy'.