In "The Lady of Shalott," no information is revealed concerning who cursed the Lady, why she is cursed, or how long she's been cursed. The "history" of the curse is left ambiguous.
This means, of course, that those details have nothing to do with what the writer is revealing in his work. Whatever is on Tennyson's mind, "who" curses her is not necessary information, so as readers, we shouldn't spend much time on it. It's not a part of the work of art. It's irrelevant to what the work of art accomplishes.
What's signigicant is that the Lady seems to be completely contented and happy fulfilling her role as a separated artist, until the song of Lancelot draws her to the casement to hear and see for herself. Whatever else is going on in the poem, a rational explanation of and history of the curse is not.
"The Lady of Shalott" was loosely based on the Arthurian legend of Elaine of Astolat Who died of unrequited love for Lancelot. Here in Tennyson's poem ,however , the lady is condemned by a mysterious curse to weave a magic tapestry. Infact the curse is more symbolic than sentimental mediavalisation. Though It is undefined it might be the gulf beteen the artist and his aesthetic distance . Now, who cursed the lady is simple to answer as none but her fate. Here too no definite answer is given.
TheLady of Shalott's curse was bestowed on her by society. We must remember that in the Victorian Age, a prevalent systm of thought was that artists must remain aloof and detached from th common quotidian lifestyle. Also, an intermingling of th two will lead to calamity. Hence, the curse itself pre-supposes death, not just death of the individual but the death of artistic flair. Considering Tennyson's stature as the representative of the Age, we can assume that he voiced the concerns of his contemporary society.