Sorry for your loss. As far as the dead hen, if you want to find out the cause of death, keep her body cool and send it to the state poultry vet for a necropsy today. Delivering the body in person is the least expensive. Here is a list of all state poultry vets contact info:
https://www.metzerfarms.com/PoultryLabs.cfm
Do you have a rooster? They grab hold of the neck feathers when they breed. Some hens will peck out feathers on the roost or while they are inside the coop. Spending somemtime with them or using a web cam for a few days might help to identify a problem.
Are you getting baby chicks? I do not like adding chickens from other flocks to an existing flock, since there is always a chance of bringing in diseases. Buying baby chicks in the spring would be the least risky choice. But that is up to you if you want to have more hens. Many people thin out their breeding flocks in the fall so they don’t have to feed extra chickens, so you might find some locally.