Sounds like there was a pecking order change, and the other hens realized she wasn't the boss anymore. It'll likely be hard to get her back closer to the top. Check her really well for sickness, make sure she's getting food and water, a sectioned off area in the coop might be the best idea for now, so that she isn't completely separated from them.She is the oldest hen in the coop though, she`s been here longer than anyone else :/ Or I would do that.