I would keep a bit of iodine on the exposed area to keep any infection out, and keep her in a clean and warm place. I would also keep her away from any other chicks if she is weak, they might cause more damage. Can she hold her head up, or does her neck flop around? Do her eyes seem dazed or alert?
just looks like a yolk sac, not hard to take care of pretty simple. Put her in a small cup with a warm damp paper towel it will dry up and fall off. Feed har and give her water. after or if she does peep up a little and it looks better put her in with the rest and see how she does. Best of Luck!