It's not possible to tell you what's wrong with your hen from looking at a picture. All I can tell from the photo is she's very beautiful, and it makes me sad to hear she's sick.
I can give you a few possibilities. The most common cause of a hen feeling sick is a crop disorder. Is it evening now where you are? If so, make sure this hen doesn't have food and water tonight or in the morning until you have a chance to check her crop.
Feel her crop located slightly in the right side of her chest. If it's fat and full when you check it tomorrow morning, bingo. You have a hen with a crop issue. Next you need to decide if what you feel is spongy and soft like a water balloon or hard and lumpy like a sack of feed. Depending on what the state of her crop is, you will either be treating for sour crop (yeast infection) or impacted crop (constipation).
Here is an article to read. Read very carefully because it has dire warnings about what not to do which can cause immediate death. So take your time with it. The treatments are safe as long as you follow directions.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
Another possibility is the hen is starving because she is being bullied by other chickens and can't get enough to eat. This is more common than most people think. But to find out if this is your hen's problem, you need to pull a chair into the run during the day and watch how the others treat her, especially when she wants to eat from the feeder. If she's being picked on, there's a fix for that, too.
https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/
Other possibilities include a reproductive illness such as ovarian cancer or infection in the reproductive tract. But you can't know for sure if this is her problem, and even if you did, most often nothing can be done to make her better.
There is the possibility of worms. Have you ever wormed your flock? There usually isn't any reason to unless you see worms in the poop or you've had a vet run a test on a few poop samples to see if there are worm eggs in it. This is an easy, quick, inexpensive test most vets will do even if they don't treat chickens. Call your vet and ask about a fecal float test.
These are a few of the most common chicken issues, not a complete list. But it should be enough to get you started. I hope something clicks and you are able to make your hen better.