Welcome to the forum!
Glad you joined us, just wish it were under better circumstances
I have not experienced exactly that so I'll just throw out a few ideas. With it coming on suddenly it sounds like an injury more than a disease, but my first suggestion is that you look up Marek's. It is a disease somewhat like cancer except it is highly contageous. Marek's causes lesions to grow at various places, maybe the joints, maybe internal organs, just about anywhere is possible but specific forms or Marek's favor the lesion growth at certain places. For example, here it is most likely to affect legs and wings where other places the neck is more vulnerable. Depending on where the lesions form, it may not affect the bird, it may cripple it so it can't eat or drink, or it may cause the body to be unable to function. Death, if it occurs, is often due to thirst or starvation. Marek's may decimate a flock or it may just affect one or two of them. But it is extremely contageous. If one has it they all have it and will be carriers for all their life.
I suggest you look up wry neck. From what you described, I'm pretty sure that is not the problem, but you asked for ideas. With wry neck, the head is twisted around but the bird can still get around.
My guess is some form of injury. I have had a hen that injured her neck to where she could not eat or drink, but she could get around a little. She did not want to though and just stayed hunched up in the coop. This sort of sounds like what you are describing but with differences. I think mine hit the feeder or something on the way down from the roost, but she may have been trying to get away from an amorous rooster. Mine could not eat on her own, but I fed her some hummingbird syrup and some water with a medicine dropper. The idea was to keep her hydrated and to give her the energy to want to eat and drink on her own. Mine was eating and drinking on her own the following day and made a full recovery.
Another thought is that she may have eaten something that paralyzed her, though why it would paralyze her legs and she would still be able to eat and drink if food is present is kind of a mystery. It's only an idea.
Good luck with her and once again,
