She is alone and has chosen a new life and she has left the past behind. You won't find the girl she used to be.
Now keep in mind that this song was written for Disney's new animated movie Frozen. In the scene where Elsa, who has magical ice powers, has run away from her kingdom because her powers have been discovered and built an ice castle to test her abilities to the limit. Here she is saying that it doesn't matter, Elsa has put the past behind her and is building a new life.