I originally heard to give her all I could get down her, so everything she put in her mouth, all day every day, had vitamin B in it. I got the capsules and dumped the powder in her water and put more powder in her food. She was isolated in a cage by herself and had no access to anything that didn't have the powder in it.
Mine had been down for only a few days when I started. If yours has been down for several weeks, it might take longer to get good results. My hen showed absolutely no other signs of being sick at all. She was alert, kept trying to stand up, and seemed to eat OK. I'm sure it wasn't as good with the powder in it, but she ate most everything I gave her anyway.
Just try giving her a lot of the vitamin...make sure it is the complex, which includes several different vitamin B forms...Thiamin, Riboflavin, Folic Acid, etc. I got this from a local Texas grocery store, but it is put out by Nature's Way. I assume there are plenty of others, but I just grabbed this and ran with it, hoping for the best. The bottle says it is for "energy and nerves".