How to move on, on a broken friendship?

[Babyface]

Well, there's nothing much you can do. If your friend doesn't wants to be close to you, you can't get close to her. I don't know what mistakes you did or what misunderstandings happened between you two but if you know there's a reason why she is being that way, try to talk to her about it and take responsibilty for it. However if she is unwilling to speak then you can't expect her to reply to you. If you have her e-mail address or her phone number, send her a message to her to apologize. Or just tell her directly. It's up to her to forgive you or not. You just do your part of apologizing to her and leave the rest to her.

People can change as time pass by. So it's not so surprising that your friend changed. It's part of human nature to change. It's not necessary that we'll be same in the future.

If she is still not talking to you and is still avoiding you, forget about it. You can stay in contact with her but I don't think you can be close to her again. So forget about being her best friend again.
Maybe you'll get a new best friend if you look for it. Make some new friends or hang out a little more with your current friends for a while.
There are some friends who leave their old friends when they get a bf/gf or new friends. Maybe your friend is like that?
Anyway if she is not interested, you shouldn't be interested as well.

Take care.