In the first episode, A Tale of Two Sisters, Elsa discovered Anna’s necklace. She found it in Mr. Gold’s pawnshop. This triggers a lot of anger from Elsa. She put the entire town of Storybrook on lock down until she finds her sister.
By taking things in her own icy hands, this has some dangerous consequences for the residents. This is especially true for Emma.
Check out the sneak peak for White Out: