Sometimes comforting someone is all about just being there and listening to whats gotten them down. The simple process of being able to talk about things can help a lot.
Holding her close and letting her know that you are there.

I agree with Ellynn, your GF needs to be more verbal and let you know what the problems are. Only then will you know how you may be able to help her.

Things like prom are typically stressful as everyones trying to look there best and they are a right of passage. So when things didn't go the way she wanted, it probably felt like one of her dreams was falling appart.

HR.