Welcome to BYC. Can you post a video of her if you stand to her up? It would need to be posted to YouTube with a link posted here. Pictures instead, would be okay. Can she use either leg, was one weaker than the other, did she hold one up, and have you noticed her ever walking on her hocks or having curled under toes? Mareks is usually considered when a chicken is lame or cannot walk, but she may have suffered an injury, a vitamin deficiency, or another cause. Sorry that she is lame. Does she eat and drink?
A lame chicken can be placed in a chicken sling to get them upright and in front of food and water. They can spend time in there, but take them out to sleep or to stretch often. You will find some examples in this link:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
Here is a good link to read all about Mareks:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/