I have had two similar cases recently both with different outcomes, both hens are just over 2 years old, they became ill at different times but both had pale combs, hunched up generally looking quite pathetic not moving around much, one was in a pen with 8 others and a cockerel, so I removed her and put in a pen with the hospital cases (ones that need a bit of TLC for various reasons) these are all on cider vinegar in the water one week followed by a tonic in the water on the alternative week. It took a bout 3/4 weeks but she did pick up and is now fine although not laying. The other was also moved to the hospital pen but rather than recovering when we went to put them away on Tuesday night she was laying on her side outside the hen house, needless to say I put her out of any discomfort straight away.
I have kept poultry for over 10 years and find that unless there is an obvious problem, and if trying a tonic or cider vinegar does not work the chicken probably wont recover. Unless you want to pay vet bills, or if the problem is spreading throughout the flock (which will need investigating) it is probably best to cull it. I would always try a tonic etc first though as you never know!
I saw a website ad called Chickenvet, have not looked at it myself yet but it might be worth a look to see if anyone on there can help.
Dawn