Sick or bullied hens will sleep in nest boxes where they feel more comfortable. She may be suffering from one of several reproductive disorders that older hens have. They can have internal laying/salpingitis, infection, cancer or other problems. Is her lower belly swollen under her vent? Has she lost weight in her breast area? Is her tail position down? Can you post any pictures of her poops? Is her crop filling up during the day and emptying by early morning? Can you get her eating some mushy moistened feed and egg? Do not mix grit into feed, but put it in a separate container nearby.