I agree with the others responses, 14 and 15 were the worst years for me. Hormones and changing responsibilities and interests. Also moving from a child to an adult is a HUGE deal for teenagers and it can be overwhelming and confusing. Other older kids that have already made that change make it even more difficult to keep it together, kids can be so mean to each other.
Reminder her that on the exterior the other kids at school "seem to have it together" but inside they all feel the same way, confused and overwhelmed. Let her know that this is normal and that everyone feels this way at times. Work on building her self-esteem and teach her coping skills.
I will never forget high school... I was the chubby weird nerd chick, always teachers pet. Everyone teased me, the only boy that liked me was as hairy as an ape and crude; it was embarassing and humiliating. My friends were the teachers, when I looked at all the other kids in my mind they had the perfect home life, perfect hair, perfect body etc. I envied them and I hated my life and my parents for not having what I thought those kids had.
It wasn't until my senior year when I was tutoring the cheerleaders that I realized they all had issues, some had eating disorders, one was an alcholic, several had parents that were never home or involved, some had been date raped. I realized then that I was ok with my so-so (in my head) existance and that my life wasn't so bad after all. I felt sorry for them and realized I was ok with who I was. I also realized that the rules and involvement of my parents saved me alot of the pain that these girls had suffered.
So my suggestion is spend time with her, do "adult" things with her. Discuss openly and frankly issues like drugs sex peer pressure etc. Don't preach, just talk about it like you talk about the weather or the blouse you bought at the mall yesterday. Confide in your daughter, tell her your worst fears and secrets just like she was your best friend. Next time you open the door and she's crying just listening to a sad song, sit with her and listen to the song also and have a good cry, we all do it, or take her downstairs and say.. if you want to have a good cry watch this... and pull out Steel Magnolias or something and grab a blanket and box of tissues and snuggle and cry together on the couch.