The head angulation is known as opisthotonus, which can be characterized by backward arching of head and neck; several etiologies have been noted, ranging from infections, viral diseases, hypoglycemia, neurological damage, toxicosis, and nutritional deficiencies, more specifically thiamine (B1), and vitamin E.
For now, house the duckling in a warm quiet area, that's softly padded and has shallow water dishes to prevent aspiration. The other ducklings may pick on her, so watch to ensure none of that behavior is occurring. Starting the bird on a broad poultry vitamin such as poultry cell, or Nutri-Dench would be beneficial if the problem is related to nutrion. Dripping a drop on the side of the duckling's bill, or soaking a cotton swab in the mixture and rubbing along the bill would work.
Hopefully, you would see improvement in providing a broad range of vitamins and supportive care, but if you don't, sadly we are limited in the amount of treatment we can provide with such young birds, due to their small size, and fragility.
