Help! The other hens have caused head injury to my sweet hen!
First you need to stop the bleeding and not let blood get into the nostrils (if blood does get into the nostrils, get it out quickly). Try using Quikstop (you can find it at a local pet store) to get the bleeding to stop. To apply Quikstop, dip a q-tip in water, then in the Quikstop, and apply.
You will also need a seperate cage for her to reside in (a small-medium dog crate will do) with food and water inside the cage (Can be a cup hanging from the side for water, and for feed use a small rabbit feeder). You can also put pine shavings on the cage floor for the chicken to rest on.
The chicken will have to be segregated from the flock for around a month, possibly more. This is enough time for the comb wounds to heal and develop new skin around them. She also may not be able to return, considering that her comb has fallen off completely. In that case I am unsure of what to do about this hen.
Oh, and while the wound is healing I recommend twice per day disinfection of the wounds with Colloidal Silver (Soverign Silver). You can get a spray bottle of the stuff and spray her head generously with the stuff.
(You already have a box..just make sure there's enough food and water while there's still space for her to turn around without knocking anything over).
What breed(s) of chickens are in the flock? Some breeds have more aggressive tendencies than others, so perhaps she may need to go to another flock? I had a similar incident except my chicken had injured herself while scared (never show chickens odd things in a confined area like tarps) and sliced her comb in half. However she was near the top of the pecking order and, consequently, was able to be re-introduced a week early because she was able to stay with the others (put dog crate inside the chicken coop) and remain in touch.
Here I don't see much hope for re-introduction.