I had a pullet die around her first birthday. She was hanging with her flock roosting on a low branch in their favorite pine tree in the afternoon. When I went to lock them up in the evening, she was found lying under the same tree, no marks or signs of illness. It can be a common thing...
Sorry for your loss. If you can retrieve her body, rinse it off, and place it in a cooler on ice wrapped in garbage bags, you can have your state vet perform a necropsy to look for what killed her. Here is a list of state vets to contact during business hours...