Breaking up is hard to do at any age, but for teenagers it can be even harder. With social pressures and not wanting to look "geeky" for being single, teenage girls can become wrecked from a breakup. As loving parents, you probably want to help your daughter get over this, for her sake as well as yours (you probably can't take many more days of endless tears!). This is a sensitive topic for most people, so tread lightly.

[edit] Steps

Discover the reason for the breakup. After all, wouldn't it be a shame to end a positive relationship over a simple misunderstanding? This is best done by listening to what your daughter has to say and providing feedback.
Sit her down and compliment her about her hair, eyes or anything. Ask her what she is feeling and why is she crying.
If the relationship can be saved (i.e. problem was a miscommunication), don't resolve the problem for your daughter. Teens hate nothing more than when a parent interferes, and they need to learn to sort out problems by themselves. Instead, talk it over with her. Help her explore the different options that the situation yields.
Let her know that there are many, many boys out there that would love to date her. Tell her that she is beautiful and any boy will want her and tell her to wait and see.
Make sure she knows you love her and that you will always be there to help her, no matter the issue: boys, school or girlfirends.
Be available to listen in ways that don't make her feel you want to control her feelings. You know when she is most likely to talk -- first thing in the morning? in the car? before bed? -- so make sure you're around then and available to listen.

[edit] Tips

Getting over a relationship is sure to take time.
Make sure she knows you care. This is a very sensitive topic, so show care and understanding at all times.
Take her shopping, get her nails done, take her to a hair salon. Make her feel good about herself. But also remember to talk with her about anything.
Compliment her and tell her she is worth anything, she is attractive and is worth anyone's while.

[edit] Warnings

Don't pry. If you know that you daughter is uncomfortable talking to you about this, back off a little. She may open up to you soon. If not, let her learn her lesson in her own manner. SHE WILL LEARN!