How did the curse fall upon the lady of shallot?

Nobody knows how she got cursed as the poet included in his poem ("She Knows not what the curse may be") She Died Because If She Looked Out the window her curse will come upon her , and she saw a knight named Sir Lancelot , who lived in Camelot , which she thought was handsome and decided to look out and the mirror started to crack from side to side, ("The curse is upon me cried" The Lady of Shallot,