Welcome to BYC. Is she eating and drinking? Alert and active? If yes, I would pick off any dried droppings, and trim any excess fluff. A wash of her bottom may help, but keep her warm until she is dry or blow dry her. I have one or two that seem to always have a problem with a messy bottom. Sometimes diarrhea for an unknown illness can cause this.
I routinely will give my hens some plain probiotic yogurt mixed well with their usual chicken feed a couple of days a week. You can mix in some water too, and they usually like that. It may help with intestinal health.