Just tell her that you are really sorry, don't know how this happened but you lost your "love" to her. Don't say that you want to break up - just tell her that you don't feel anything anymore.
And:
If she is smart and cool girl - she will understand, probably will offer being a friends
If she is not smart and cool enough - God bless your soul.
I had this situation 3-4 months ago, my luck my crush is smart and cool enuf.
And I agree with some words by Kitfox - no sense to pull this relationship, if one side is not interested.