Sick Chicken! Need help

My husband went out and fed the chickens this morning and everyone was fine. We went out for their late afternoon check and one was sitting in the corner of the coop (outside in the cold) and wasn’t moving. We thought she was dead but then opened her eyes when my husband picked her up. She’s now inside under a heat lamp and we were able to give her some electrolytes with a syringe and she drank. We have checked for eggs and don’t feel anything and don’t see any mites or lice. What else can we do for her?!  Any advice?

1. Approximately how old is this chicken?
2. Does she seem "paralyzed"? If so, Google Merricks disease - if there's *any* chance it's Merricks, keep her isolated as it's contagious.
3. Did these birds get any immunizations?
4. Is there any chance some sort of predator got in there? Poisonous snake, or flying predator. Things like that can shock a bird and warmth and fluids is the treatment, as you've done.

Keeping her warm and quiet is step one. We lost a bird recently to #2 above and it sounds somewhat similar and there's no cure for that, but we've also had birds with strokes, shock or injuries that have recovered amazingly well, so Step Two is to watch and wait.