That is really strange indeed. Has there been any other change in your flock? A change in environment, or a change in birds? Have you lost any birds? Sometimes when there are environmental changes or a bird dies or is removed from the flock, the pecking order will change. Your aggressive hen is...
Do you have pictures? Was the aggressive hen familiar with the attacked hen, or was the attacked hen a new addition to the flock? I would flush the wound with a saline solution and put iodine on the wound it keep out infection and wrap her neck with gauze (don't wrap the neck too tightly, you...