Board Thread:General Discussion/@comment-24589671-20141027013232/@comment-25602483-20141105005543

The most likely explanation I could think of is that somehow lily came back for emma, but knowing that Ingrid and Emma had become close, she was a threat to the SQ and Emma ralationship. The SQ then tried to get rid off or succefully got rid of Lily, but Emma found out about it which probably ended their good relationship. So the Snow Queen erased emma's memory about their connection and waited until season 4 to reintroduce herself.