Welcome to BYC - so glad you've joined us, but sorry it is due to the circumstances you are facing with your hen.
When you say her comb is missing is the entire comb or is it significantly damage but still there at least partially? Is the flesh beneath the are the comb was attached damaged? It may help if you provide photos of her wound.
Chicken society and behavior can be shockingly violent at times - and the reasons aren't always apparent. Some of the behavior of a broody hen can be similar to that of an ill chicken, and a weak or ill chicken is often the target of aggression from flockmates. Sometimes the broody's aggressiveness can incite a poor reaction from other birds in the flock. That being said, did you witness the attack or are you assuming that is what happened to cause the damage she has?