I'm pretty new here but wow! That looks painful! From everything I've learned on this site, it appears she had an injury of some kind but that the others chicks have also been pecking at her too. I would definitely separate it and others will be by shortly to tell you what to do to treat it. Good luck!
Separate her, clean the wounds with weak Betadine, and put Neosporin on the dried off skin. Get some BluKote spray for when you put her back in after she heals a bit. This chick has major injuries to her throat, and it may cause respiratory distress or difficulty breathing. I would feed her some vitamin/electrolyte water for a day or two, give her some eggs or moisten her feed to make it desirable. I'm not sure she will make it, but I sure hope so.
Are they outside in a brooder or inside your house? I see chicken wire in the picture, and I would wonder if they are outside or in a coop, if a predator such as a rat or weasel could have gotten in? Are any of the other chicks older who could have attacked her? If they are outside, I would move them inside.