Can you post a pic? I had a duck like that once, she actually didn't have a broken neck. Does she have any injuries or is it just sore? If she has injuries on her neck use antibiotic cream on the wounds and cut off the feathers. When my ducks were like that I kept them in the house and bathed them every day in shallow warm water then toweled them off. I'm not sure if the chicken would appreciate the bath though. Mostly I would say just try to keep her warm, dry, fed, and clean. You can gently touch it to see if it feels off, most likely it's sore or has a pulled muscle or something of that sort, although it can still be very painful. Try to get her to eat anything. Offer treats, fruits, veggies, anything to get her eating. To get her strength and immune system healthier and give them a boost you can put probiotic in her water/food and if you have it on hand you can give her Elderberry syrup. Anything that's natural is good for them too. Also, if she's stresses you'll want to calm her down so put herbs like lavender in her brooder/coop/crate.