I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site.
Unfortunately, the symptoms you've mentioned can indicate any number of illnesses or health issues. In avian medicine, there's rarely one cause of a condition, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. With this in mind, your best course of action is to reach out to your county-extension poultry personnel or avian-oriented veterinarian (please see here: www.aav.org) for help in differentiating the various causes of what you're seeing. Veterinarians can perform a physical exam and run diagnostic tests, including X-rays, to distinguish between the various etiologies.
It's best to approach the diagnostic process with a clear sense of the bird's financial value to your operation. Although some services such as your county animal disease diagnostic laboratory might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one hen. It's particularly important to sacrifice a very ill bird or refrigerate (not freeze) a newly dead bird and have it necropsied if you have a sizeable flock. This is often the only manner in which you can know why your ducks died and how the rest of your flock might be addressed.
Please let me know if you know of any other worrisome symptoms - vomiting, diarrhea, sneezing, coughing, nasal discharge, increased respiratory rate, lameness, etc.