My hen died

I never really figured out what was wrong with her or get a lot of ideas. She was the runt, she was the only one of her kind, and she was picked on by the others. She had trouble the past week 1/2 walking- she would fall over, or fall forward. She would stay puffed up and neck tucked in. She would eat and drink when she was inside alone- away from the others. She finally died today. She had been sick for about a month. Wish I knew what caused it. Thanks for those that did respond to my previous posts.