You may find that just removing her from the flock to a warm place and making her feel safe is sufficient to revive her with a bit of food water and time. Sometimes a warm Epsom Salts bath will help revive and relax them. It makes it easier to do an internal exam if they have had a nice soak and are clean. Learning how to care for or dispatch sick chickens is an important part of poultry management as unfortunately there are a myriad of diseases and ailments that chickens can suffer from. It is one of the things that brings many of us together here on BYC, learning to diagnose and treat our feathered friends or help them to depart this world if we can't fix them. I have used long handled loppers or pipe cutters to euthanize sick birds or cervical dislocation (broomstick method). Do be aware that they will usually flap a lot afterwards, so be prepared for that. It will usually last a minute.... which seems like a lifetime. If they are very sick and weak, it is less bad. Be assured that it is just after death nerve impulses and common in many animals but particularly long and distressing in chickens..... the phrase "running around like a headless chicken" was not coined for nothing and really can happen albeit uncommon, if they are not restrained.
If you decide to try to treat her, we are here to help you but we will need more information..... age, was she actively laying recently, how is her body condition.... feel her breast.... is the central bone sharp under the skin or reasonably covered. How does her crop feel?.... that's the pouch on the front right of their chest where they store food. How does her abdomen feel... particularly between her legs and around her vent. Any abnormal swelling in these areas? Compare to other chickens by feel as feathers can hide a multitude of problems. How does her vent look like? Are her feathers soiled with poop? The bloody comb will be due to pecking but she has most likely been victimised because she was already sick. Usually they look far worse than they at first appear. Once you get her cleaned up, you will have a better idea. Even chickens that have lost an eye can recover with a bit of TLC.
Good luck. Whatever you decide will be the right course of action and we are here to support you in any way we can.