Actually, I fixed it, but am not sure why it had the "!" in the first place.
I went to the external hard drive, Time Machine back up, and found it there.
Was able to play it from there, then just dragged it to the iTunes icon on my desktop.
As I said, I have no idea how this song got there. It's a case of having 2 of the same song, one with vocals and one with accompaniment. The accompaniment version was there, but the vocal is the one that was messed up.
Not sure if there are other songs in my iTunes with the same problem or not---sure hope not!
Thanks so much for your quick response, though!