Looks like a back burn from a rooster to me. If you have a rooster, either confine the hen so she gets a break til her feathers grow back out, about three weeks, and/ or get her a hen saddle, they are very affordable, available on here, and cute, too.
If you don't have a rooster, if she were mine would put a few drops of cattle ivermec on her at the base of her neck, or treat her for mites / lice with something similar. And after that make sure the whole flock has access to dust bath and you can toss some sulfur or ashes or Insectrin powder in there too. (no need to use more than one)
And you could put BlueKote or NoPick on the bare spot. It is possible that the flock is picking on her, or that she's pulling her own feathers, which they sometimes do from stress or mites. In which case, the saddle should help with that too, and/ or the Bluekote or Nopick. If you need any more info, though, I'd suggest you repost on the injuries/illness forum, so more people see it. And if you do confine the hen, make sure she can still hear and see the flock, so she doesn't get stressed.