theres alot you can do. hug her, and make her feel comfatorable. keep asking what is wrong untill she answers. see, we are girls, our hormones go crazyyyy! soo, sometimes, we may feel that you dont love us or w/e you know? im a girl, and i go through it all the time. just make her feel that you love and care for her.
I think it would make her feel better if you started playing video games, or take a nap or have a few beers with your friends. Just kidding, actually just do what the first poster said, unless it's your fault...in that case you grovel and beg for forgiveness.
Be there for her, let her cry on your shoulder and just hold her. If she wants to be alone, don't fight her. You tried to be there for her and she appreciates it. She just needs time to cry for whatever reason. and like Timaaa said, she will call you when she is ok and ready to talk and be herself.
It's not your responsibility to make someone feel happy or better. She might have a chemical imbalance or a mental disorder in which case you need to take her to a professional. You're setting yourself up for more if the same if you comfort her when she is acting crazy or not normal as she will learn that her behavior gets a response that she likes from you.