Are you in the US? Was she vaccinated against Marek's?
What is her standard feed routine including treats and supplements? How hot has it been where you are?
What does her crop feel like.. flat, swollen and squishy like a water balloon, or a hard knot inside? Is there any smell coming from her beak?
How are you feeding her now.. when was the last food she had? What's her droppings look like? (okay now I see she just got THIS bad this morning)
Highest possibilities currently in my mind without more information.. severe dehydration/electrolyte imbalance, malnutrition, blocked/sour or other crop condition (possibly causing the fore mentioned conditions), ultimately possibly Marek's which can present in a whole host of ways including only individual birds and with visual changes..
Other considerations.. internal egg issues.. excess internal parasite load.
The eyes closing.. is most likely a symptom of lethargy. However, you did mention it as your first clue.. are you able to tell if she can still see or if the vision seems to be changing, was she still pecking at food before being this bent over? Try to get a look at the pupil shape and iris color.
There may be a few more things that can present this way.. try to start answering some of my questions and I'll tag another knowledgeable poster who might have more or different helpful ideas/questions than mine. TIA
@azygous
Depending on feed routine (possibly even storage conditions).. supplementation *may* be helpful.. These sort of symptoms *can* be caused by E and thiamine deficiency..
https://blog.meyerhatchery.com/2017/08/wry-neck-in-chickens/
Be aware.. once it's gotten this far.. it usually takes many days to weeks to see improvement IF supplementation does help, and may require some physical therapy to ever return to somewhat normal. There will not likely be an immediate full recovery, unfortunately. If you're unable to support her through this.. then culling/euthanasia may be an option.
I'm not sure if necropsy (including microscopic tissue slides) will show cause, but it may and may also be useful to managing the remaining flock.. in which case if that time comes, refrigerate her body, and some contact information for the state labs is in the following link, free or affordable to backyard keepers in many states..
https://www.metzerfarms.com/poultry-labs.html
Still, I hope she recovers quickly!