Sorry for your loss. When you lose a chicken, it always is a good idea to have your state vet lab perform a necropsy, and testing for Mareks if it is indicated when you lose one. You would keep the body cold, wrapped in 2 clean garbage bags, and transport it to the state vet. They also send a...
Have you been with your hen this am? Is she acting any better? Has she taken some fluids or electrolyte water? It is good that you have noticed something wrong. Is she low in the pecking order or possibly kept from food and water by the others? Could she have been injured by a rooster or other...