She may have an injury. Is she with others her age or any older chickens? How long has she had the problem? Does she eat normally, or have any diarrhea that has made her very weak? Have her legs always appeared normally up till now? What are you feeding? Can you place her next to food and water, and think about making a chicken sling to get her upright to rest her legs? Here is some info on slings: https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
I made a sling. She’s doing better. Looks like her wing or leg is injured. Is this something that will get better or does it need to be assessed. Thanks you