“Standing frozen in the life I've chosen
You won't find me, the past is all behind me
Buried in the snow”

Demi Lovato – Let It Go Lyrics on LyricsMode
EXPLANATION
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.
EXPLANATION
She keeps her powers unknown even to her sister.
But now every one has discovered Elsa's power because of Anna. She has taken Elsa's glove and that's why everyone know's Elsa's most deepest secret.
EXPLANATION
It fits so perfectly with the movie and when it comes to the part where she sings "let it go", you can feel that she's just kinda trying to release her powers and just letting herself flow. It also kinda means that she's not going to go to prison (watch the movie) and she's gonna stay inside her Ice Castle she made (see the Let it Go Music Video) and she's going to stay there.
EXPLANATION
She changed her life, And the past is still behind her but she forget about it to start a new life.
EXPLANATION
We should not afraid to let go of our past and start s a new life for ourselves by moving forward not looking back, highly recommend you watch the movie.