I wish I was knowledgeable enough to give you advise but I'm not. I never thought I could love a chicken I have 4 I think I have 3 hens and I Rooster but still don't know after 2.5 months. I love holding the one named Ethel, she or he is so sweet to me. The others just want their food and water...
Maybe she stuck her head in or through something and injured herself. I check around to see if that may be a possibility. I wish her a speedy recovery.