I bought a song, played it, but now it has an exclamation by it and says it can't be found. when I search my computer it says no results. when I go to Itunes it shows it as a purchased song, but can't play it there either or re-download it!!

I bought a song on July 25 of this year 2015, played it and it worked fine. But now it has an exclamation by it and says it can't be found.
It asks if I want to locate the song. It takes me into my computer and it's not shown. I tried to search for it and it says no results found.
When I go to Itunes it shows it as a purchased song, but can't play it there either or re-download it!! Again it gives me the message that

it cant be found.
I can find no way to re-download it. - I went to Purchased - and its there, just no way to redownload. I'm stumped and cant find help on itunes