After she hears Sir Lancelot singing, what does the lady do?

goes to window and looks out

She looked through the window instead of looking through the reflexion on the mirror

Explanation:

She left the web, she left the loom

She made three paces thro' the room

She saw the water-flower bloom,

She saw the helmet and the plume,

She look'd down to Camelot.

Out flew the web and floated wide;

The mirror crack'd from side to side;

'The curse is come upon me,' cried

The Lady of Shalott.

This is the excerpt of the poem were it is described how she ignored all warning and look at the world as she was not supposed to.