How do I cheer someone up who's dealing with a tragedy?

My girl friend this time last year was involved in a terrible situation that involved one of her close friends killing himself on webcam with her. She’s been really depressed the last few weeks because of that. She wouldn’t tell me why she was sad (one of her close friends did). I really want to try and make her feel better, or at least be there for her to maybe help through this hard time. But because she didn’t tell me and still won’t I’m unsure if she wants me to be there for her… I’m afraid if I start talking to her about it and try to comfort her it might backfire because she didn’t want me to know about it. I’m really worried because she has a bad history of depression and has even been hospitalized before… what should I do?