My itunes doesn't recognise an album purchase but the itunes store does?!

Ok, so I wanted to listen to a praticular album today, went to look for it in my itunes and it wasn't there. So I went to the itunes store (because I remember buying it) and there it will allow me to play the whole album and states 'purchased' next to the album title. So why does it not show in my itunes and why can't I download it? I have checked for all available downloads. What can I do to get this album back in my itunes library?

Thanks in anticipation.