I would suggest moving the duck either inside the house or make a pen inside the coop that would allow her to easily access the feed, and water, and where you can monitor her more effectively. Try offering her warm electrolytes, and fluids if she is not drinking much.
What you have described sounds like she may have an egg stuck in her oviduct which can create pressure on the nerves surrounding them, and result in short-term paralysis. The onset of this condition tends to be quick and will happen overnight.
As of now, that would guess without any sort of additional information.
Is the duck a current layer?
When is the last time she laid an egg?
Has she had egg issues in the past?
Is the duck currently eating, and drinking?
How do her droppings look?
Are there any additional symptoms worth mentioning?
If you have not already, I also suggest a thorough physical examination if her condition allows you to. More specifically I would be concerned with her abdominal area (distended, hard, etc), how her cloaca looks, and a look over both legs.
