Chickens doe sometimes attack each other, seemingly without reason, and will take it to the point where they kill the bird. Seperate her. Put antibiotics on the comb (not neosporin pain relief! normal neosporin is fine.). Then, if it the comb has stopped bleeding, put some blukote on it. It tastes bad to chickens.
Also, sometimes, if there is wire fencing, and there is something they want to get to on the other side, they can damage their heads on the wire (think cheesecutter). Still, the other chickens may be fascinated by the blood and go cannibal, so still follow the above intructions.
Good luck with your lady!