From what I remember, the girl and her family moved, and at their new house there is a woman who is a witch of some kind. When the girl discovers this and goes to find out more, she is somehow transported to Atlantis (which I think has something to do with the mirror... maybe).
When she gets there she meets a guy and forms a friendship and more, but in the end he dies in the sea and she is transported back to her world. There are around three books in this series and in one of the books she is climbing a tree of death, with the heads of people growing from it (I think she finds her Atlantian boyfriends head?).