Lack of eating and drinking is very serious and is something that needs to be addressed before we even begin to think about the underlying cause of her problem, her current state of health needs to be the first priority here, IMO.
I don't believe anyone has mentioned it yet, but is a vet an option? If so, that'd really be your best bet here.
Here is a Waterfowl/Poultry vet directory,
https://www.metzerfarms.com/Veterinarians.cfm?CustID=27014516
If you are unable to find, or take her to a vet, you'll need to start gathering tube feeding supplies so you satisfy the bird's nutritional and fluid requirement. It's important to note that most sick birds tend to die of dehydration before anything else, that's why its so important we take care of this now.
For tube feeding, you'll need some Parrot baby feeding formula (Kaytee Exact is great), a few 35/60ml catheter tip syringes, you'll be able to find the syringes at any feed or medical store. A gram scale to weigh her on, and lastly, some tubing, which will likely be the hardest thing to find, preferably you want a catheter, but you can also use plastic aquarium tubing and meltdown the tip to make it smooth.
Once you get those supplies we can proceed further, and you may consider reading some of these links regarding the topic of tube feeding birds,
https://www.backyardchickens.com/threads/tube-feeding-q-and-a.1215032/
https://www.backyardchickens.com/th...ng-guide-pictures-under-construction.1064392/
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
Sick birds often have a "tucked-in" head looking appearance, but if she's physically unable to pull her neck out/ or keep it steady, that may be due to the paralysis of the anterior neck muscles. This can be caused by several things ranging from vitamin deficiencies, toxins, viral or bacterial infections, to neurological damage. Without the diagnostics tools a vet has on hand, diagnosing these conditions are somewhat hard, but we can narrow down possible causes by treating for some of them, and looking at the bird's past history, and using some of that knowledge to base off of which condition would be more likely in the long run.
So that is something that we can look into, if she does pull through, but right now we really need to focus on getting her hydrated, and back into a somewhat stable condition, so we can move on further with this.
As others suggested, a video of the duck would be quite helpful.