Any chance you could get her from the roost and take her inside or to a well lit and safe area to see how she's acting in an environment where she's able to see well?
How old is she? Is she eating and pooping? What breed is she? Have you checked her over really well, getting under those feathers to make sure she hasn't been injured? Is there a chance she's had a fall? What is she fed (including any and all treats)?