Welcome to BYC. We need to have some details about what is going on with your chicken. How long has it looked like that, does it look like a skin infection, or has the skin been injured? If this just occurred without an injury, I would at something like a skin fungus called favus. Has she rubbed her head on fencing or been bullied? I would also look for mites or lice around the neck and under the vent.
Favus is a fungus that should be treated with an antifungal cream such as clotrimazole (Lotrimin) or miconozole (Monistat 7 cream.)