Do you have any antibiotics on hand? Anything left over from a human prescription?
It's entirely possible this hen is suffering from botulism toxin from the compost pile. Composting discarded kitchen scraps and old chicken feed may seem ecologically virtuous, but if chickens have access to this toxic "stew" while it's decomposing, it can be deadly. Over the many years I've been on BYC, I've seen any number of cases of death by compost, including in my own flock. I no longer compost anything other than horse manure.
Getting this little gal on an antibiotic immediately could save her life. If you have no antibiotics and have a family vet, contact them and ask for amoxycillin or something similar to treat Clostridium bacteria.
There is also a chance that this is mold toxicity. To be sure you aren't leaving that untreated, you can start her on activated charcoal capsules, two now, two at mid day and two at bedtime, slipped directly into her beak. Provide plenty of fresh water for her to drink.
To summarize, it's possible she has either or both Clostridium bacterial infection and mold toxicity. Treat for both. Tell the vet they don't need to treat since most will refuse anyway, that you only need to buy the antibiotic from them to treat a chicken for ten days at 250mg twice a day. You can buy the activated charcoal capsules at
Walmart where the vitamins or stomach remedies are shelved.