Hi Felicity, I am sorry to hear your hen isn't well.
From what you have mentioned, it sounds pretty much the same as what recently happened to my hen which was internal laying. While all the other girls were coming in to lay, she hadn't laid at all and I thought it might be that she was simply too old. Then I saw that she was standing on her own, all hunched up, her eyes closed and she looked ill. She had a dirty bottom which was usually a lovely fluffy bottom. I am very new to keeping hens so researched what it might be and luckily it worked for her. I am very grateful to all the people who post on various forums as it is where I got all my information from that has been fantastic.
I ran a hot shower to warm the bathroom and at the same time, a luke warm (not at all hot) bath in the sink. I mixed in a tablespoon of pink himalayan salt into her bath water as I didn't have any epsom salts and wasn't sure whether ordinary salt would cause harm – might sound silly but as I said I am new to this. I switched off the shower when the room was nice and warm. I grabbed a towel so it was ready for when she got out.
I picked her up gently and she didn't protest and brought her inside. I closed the bathroom door behind me and placed her gently in the sink. I held her in the water so that only her abdomen and underside of wings were covered and stabilised her with one hand, she sank down into the warm water comfortably and was only half covered. With my other hand, I ever so gently stroked her abdomen from the front to back, hoping that if she was egg bound, the movement would help her to move it out. I also cleaned her bottom of poop and noticed there were tiny bits in the water which turned out to be dead lice. And there was a lot of them, most of which I found near her vent. The bath helped by killing alot of them. I couldn't release the plug and pile fresh water in so I thought I'd deal with one issue at a time, the lice issue afterwards. We were in there for at least 20 mins and I just talked to her gently, telling her to relax. Her underside was hot to touch and the water was not so I thought that she may have an infection.
When she felt it was time (she started making movements to get out and chirping a bit), I grabbed the towel next to me and picked her up gently with the towel and held her in the towel for a bit, keeping in the heat. I then patted her dry as best I could and by that time, I felt she was telling me she wanted to go back outside. I took her outside and she went straight to a nest, and when I went back out about an hour or so later, she was standing there with shell less eggs x 2 dripping out of her (one was on the ground and two were coming out). I helped her along and pulled them out with some paper towel and noticed her vent had prolapsed a bit.
I took her back inside for another a bath to clean her up, the same process but this time using Johnsons insecticidal shampoo for dogs which kills lice and their eggs. I rinsed her off in some more water which she didn't like but it had to be done. After this, I held her little vent in for a bit, and waited until I was sure that it was staying put. Luckily it stayed in and after that one, she was seeming a lot brighter. It was as though she felt a release and could breathe again.
I then called our local vet who took her temperature which was normal, but she may still have had some bacteria build up from the internal laying so she was placed on the antibiotic Baytril for 10 days. During the 10 days, she and another hen who began displaying similar symptoms were given the Suprelorin implant which has given them both a break from laying.
For 3 weeks, she slept in a box in our home with nest litter, food and water and each morning, she hopped out to be with her friends and was behaving quite normal but a bit slower than usual. I think she needed that time out from the coop to be able to get a good nights sleep on her own so that she was strong and able to handle herself around the girls during the day time. After 3 weeks, she was much stronger and I encouraged her to move back in with her friends. We are now 6 weeks in and they are both doing well. They're ex battery hens so we take each day as it comes but so far, so good - both eating and drinking and scratching for bugs, dust bathing and enjoying life. I believe the warm bath was the first step to saving her life. It relaxed her body enough to get rid of at least 3 shell-less eggs. It also showed up that she had lice, something I was unaware of before and was able to treat her. The antibiotic, Baytril, was the next step that saved her because although I'm usually against chemicals, when someone is weak, they cannot fight off harmful bacteria as well as someone who is healthy can. And animals are just like us. I would also highly recommend the Suprelorin implant if you can get it – it's expensive as an ongoing method of treatment but nowadays there are options such as crowdfunding sites like www.gofundme.com so see if you can get some crowd funding perhaps? I have also heard it is not available in some countries but there are possibly some other options, depending on where you live.
I had to treat the whole flock for lice and also the coop and used smite powder which is safe for the environment and vinegar in the coop which kills mites and lice but doesn't kill wildlife.
I would start with a warm bath, if you can and separating her from the flock, keeping her inside in a box with some nesting material, food and water (try sunflower seeds, olive oil and maybe a mixed wild bird seed) and some of her normal layers pellets mixed with warm water. It's a catch because although she has gone off her food, she will need to eat to expel poop / egg and keep everything moving properly to stay alive so try all her favourite treats and hopefully she'll take to something. Some people have had success hand feeding by syringe but I would not feel confident doing it myself so perhaps check the forum out to see how it is done? And to minimise stress, if you can allow her free access between her box and the outdoors to her normal free range life if she chooses, that would be good. If the other girl is also laying shell-less, she may be having the same issue so you may wish to treat them at the same time? Of course, it could be something else entirely but as I said, this is what worked for me.
Best wishes to you and your hens.