The words to the Radiohead
song are over at Let Down
, if you'd like them.
However, in its civilian life, this word/phrase (one word or not? i won't make the call) can be either a verb or noun.
(v.): "you really let me down" to disappoint or come up short, be found lacking in the face of expectation. "I let down my parents when i killed that little girl."
(n.): a disappointment, thing which raised your hopes and then dashed them. "Well, your performance was certainly a letdown."