For me, they're no different than my cats who were free initially, but priceless really. All of them have the same hope to live long and well. Not that it's easy by any stretch - work many hours to be able to provide med care....
Anyway, to your poor hen. I really hope she can win this battle so let's see.......
Has she ever been wormed? What does her poop look like? Wormer may take an additional toll on her and if she doesn't have worms that would be unfortunate but....
Also, I normally wouldn't suggest this but if she can't receive any testing very quickly to get at what the problem is, her only chance might be a shot in the dark with antibiotic
(immediately). Which antibiotic may depend on what you can get your hands on really fast (and thus it may not work). Antibiotic, and the warmth, quiet, hydration, etc. noted above. When Ellie got very lethargic suddenly on Labor Day (the hen mentioned above) she was put on baytril - she turned around very quickly but I also acted so very quickly - and it was a stroke of luck that her problem was apparently bacterial in nature. I don't think you'll be able to get baytril at a store tho dog/cat vets offices do have it. The dosage would have to be adjusted for a chicken. If you are not going to do any of this, this bird may just suffer till it finally dies, not a good option either IMHO...so another thing for consideration is humanely euthanizing somehow. She's so young though - it would be lovely to turn this around so she could have a decent lifespan.
I wish we could all know what's wrong more easily, and how to help them. I'm hoping she can get well
JJ