authorizing a song

I downloaded a song from the itunes store but when i try to play it i am unable to do so because itunes says it hasn't been authorized. so i sign in to authorize it but itunes is telling me it has already been authorized. i go to my account, in the drop down box i click "authorize" and it tells me my lap top is authorized. i've gone through this several times and nothing changes. How do i fix this?