Unfortunately that is a pretty classic Marek's splits posture. If she is smaller than the rest, that also suggests Marek's. I have had birds recover quite quickly from such an attack after finding them floundering on the ground like that, but I have also had them deteriorate. Place her somewhere warm and safe and offer her moist food and water and poultry vitamin supplement........scrambled egg and a little tuna is good as she needs extra nutrition to support her immune system.
You might want to make her a chicken sling or hammock to support her in an upright position. Unfortunately, even if she recovers from this attack, she will most likely suffer another usually more serious one at some future date maybe months later but quality of life can be good in the mean time, if she regains some leg function. Keeping her in view of the other chickens will help to keep her from depression, so if you can make a hammock for her and put her in a cage in the run, that will help, provided it is warm enough. The others will have been exposed to the virus now, so I don't quarantine my Marek's sick birds but that is a personal choice.
Good luck with her and the rest of your flock. If she dies, it might be a good idea to send her body off for a necropsy to confirm what you are dealing with, but I'm pretty confident from seeing that photo that Marek's is the cause.
Best wishes
Barbara