In the first stanza of Part II, the narrator tells us that the Lady of Shalott has no time "to sport or play" because she must continue to weave and weave at her loom, and she is not permitted to stop her weaving. The narrator says,
A curse is on her, if she stay
Her weaving, either night or day,
To look down to Camelot.
She knows not what the curse may be;
Therefore she weaveth steadily . . .
In other words, the curse will take effect if the Lady stops weaving, and so she cannot ever stop, during either day or night, for any reason—even for something as simple as glancing out her window to look at Camelot below. The Lady does not actually know what the curse is, or what exactly will happen to her if she stops, and so she never stops.
Because she cannot stop to look out her window, she keeps a mirror that reflects what is happening outside; this way, she can look at her mirror—without ceasing her work—to see what's going on. However, when she catches sight of Sir Lancelot, she runs to the window without thinking. Suddenly, her loom flies apart and her mirror cracks, and the Lady knows that the "'curse is . . . upon [her].'"