So sorry for your loss. She was very young to have Mareks disease, but it was possible. Normally they are several months old before showing symptoms. However a serious leg bone deformity, slipped tendon, or vitamin deficiency could have caused her inability to walk. Good luck with your other chicks. Here are some good articles with more information on Mareks:
https://extension.umd.edu//sites/ex...Preventing Mareks Disease in Small Flocks.pdf
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
https://extension.umd.edu//sites/ex...Preventing Mareks Disease in Small Flocks.pdf
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/