Hi,
If you can get her to a good duck vet, that is best - and I know not always possible.
Worst case, she might have egg yolk peritonitis. That can be helped, she would need antibiotics. She may be at the point she needs tube feeding or syringe feeding. casportpony seems to recommend going with tubing when necessary, and has written a number of posts about it. This device won't tag, so feel free to contact her, or write that name with an @ sign in front, and if the letters turn blue, it will notify her.
She also could have metal poisoning - that is also treatable, and needs a vet's help unless you have access to chelating agents and know how to use them.
If she has a more mild toxicity problem, charcoal slurry may help - there is a recipe for that at the top of the Duck Forum - there is a Duck Forum, and for some reason, that's where most of the duck people are active. I apologize, it's been rather a busy time for me here lately and I just saw this post.
How is she today? I will be going to bed soon, but plan to check back tomorrow.
She might have some kind of other virus or infection. How are her eyes, her nares, how is her breathing?
Have you tried any kind of treatment yet? It is so hard to know what to do sometimes.