Botulism
Symptoms: Loss of muscular control of legs, wings and neck, sometimes called limberneck. Birds are unable to swallow.
Cause: Toxins produced by Clostridia bacteria in decaying animal and vegetable waste. Avoid problems by keeping ducks out of muddy/dirty areas, and stagnant water, especially in hot weather. Bacteria multiply rapidly in warmer temperatures with anaerobic conditions (no oxygen).
Treatment: Give affected birds fresh drinking water. If necessary, introduce water into the mouth and throat. A crop tube could be used with the advice of a vet. Add Epsom salts (magnesium sulphate) to the water. Recommended amounts vary from 1 tablespoon in one cup of water to 1 ounce per 50 fluid oz (2½ pints) of water.
I'd try Epsom salts. Keep in mind that ducks drink a lot of water, so maybe go a little more dilute than what is recommended and if she is drinking more, dilute further.