I'm sorry to hear about your hen.
Was it the same one you posted about previously?
https://www.backyardchickens.com/threads/behaving-strangely.1441162/
To find out the cause you may consider getting necropsy/testing you can find your state lab here
https://www.metzerfarms.com/PoultryLabs.cfm
The symptoms you describe in the other thread (if it's the same hen) could be from a number of things - crop issue/blockage, canker and respiratory disease are a few common problems. Gapeworm is a possibility but birds generally stop eating/drinking because they are always gaping.
You can treat your flock for Gapeworm if you feel it's necessary. Getting a fecal float is also another option that way you don't treat needlessly, especially if you don't see any symptoms in other birds.
Gapeworm can be treated with Fenbendazole (Safeguard). I would use the dose of .23ml per pound of weight given orally once a day for 5 days in a row.