Chickens are very territorial. A few months ago, one of ours was attacked by an opossum, although I'm pretty sure that the chicken attacked the opossum first. Short version is that I locked her up to recover and when I put her back out, she must have still smelled like opossum because the other birds attacked her and drove her off. The thing is that two of our more distant neighbors also have chickens. We've hosted some of theirs in the past and eventually they leave. Today I counted my birds when I fed them, they were all there, then heard a noise in the coop. Opened it up and there she was, laying. She looks good. So we're back to normal!