Probably would be the best route to go.
There are so many conditions that can cause what you're seeing, so attempting to guess over the internet is merely grasping at straws.
Could you treat with antibiotics, dewormers, and tube feed (?) probably, and that very well may work, but in a situation such as this it's best to address the underlying cause as effectively as possible, and that usually entails a thorough physical examination and diagnostic testing to establish what you are dealing with.
In the case that you couldn't get to a vet, It would be better to treat in an aimless manner rather than not treat at all.
Additionally, if you can't get to the vet today, I would suggest you collect tube feeding supplies; start off by correcting any nutritional/fluid deficit, then start the bird on maintenance fluids as need be. When a bird is not eating, they are unable to maintain normothermia thefor we should increase the temperature in their pen ( still watching for signs the bird may be overheating.
View attachment 2725219 Excluding the crop needles in this photo I took, this is merely all you would need to tube feed at home. For a grown medium-sized duck I personally use a 16-18 french size tube. If you cannot readily find these near you, you can use aquarium tubing of appropriate size with the tip melted down to prevent esophageal damage during the tube feeding process.
More information on tube feeding can be found here:
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/