Here is some reading about Mareks disease:
https://vetmedbiosci.colostate.edu/vdl/colorado-avian-health-program/mareks-disease/
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
Here are some examples and a video of chicken sling chairs in posts 5, 11-15...
At this age Mareks comes to mind, especially since you recently lost another bird mysteriously. I would make sure that she is drinking fluids and eating often. Food needs to be near her. A chicken sling chair might be helpful to get her up and help with feeding. Could she have gotten into any...