@blazinazn - Thank you for your reply, I probably didn't make myself very clear. I updated with Windows Media Player and it still didn't work. The track that won't play isn't the song that came with the phone - it's one I deleted from my SD card.
@jmorton10 & AndyCR512 - Thank you for the suggestion, but it didn't work
(
@ScrapMaker - I don't know what happened. I think it was when I synced it - but now I don't know what to do? This happened to me when I first got the phone and then it somehow sorted itself out, but I don't know how!