Any evidence of a struggle? I had a young pullet "murdered" by the older hens -- they broke her neck. (She had been ill, evidently they sensed the weakness and attacked her... there were feathers all around, no blood though. She died an hour after I found her. It broke my heart.)
If she is not too young... Roosters can also break the neck of a hen when they grab them to mate. Rape by rooster. I have also lost one hen to a broken neck by a rough rooster rapist... got rid of him immediately. There is no evidence of violence when it's a rooster rape.
If her neck is broken she shouldn't last long. I'm sorry. I don't know what else it could be.... someone else may have other, better ideas.
Good luck.
