What poem by john keats begins upon a time?

The poem by John Keats that begins "Upon a time" is "La Belle Dame sans Merci: A Ballad."

Here are the first lines:

> "O what can ail thee, knight-at-arms,

> Alone and palely loitering?

> The sedge has wither'd from the lake,

> And no birds sing."

Let me know if you'd like to read the whole poem!