What's wrong with my dog?

I have two dogs, both of whom are female. A big Saint Bernard, and smaller dog of a breed I don't reckognize. The two had a rivalry ever since I bought the Saint Bernard as a puppy, and sometimes even fought over who's top-dog. A few days ago, it escalated into my Saint Bernard biting into the neck of the small one, and shook her around like a rag-doll. She's been depressed ever since. She doesn't eat much, she's nowhere near as lively, all she's doing is sitting and sleeping quietly, while also being scared to death da Saint Bernard. I don't know what's wrong with her and I'm afraid she's close to dying.