Computer Authorisation

I have 1 song in my library which will not play in itunes or on my ipod. Every time I try to play it (in itunes) it says 'This computer is unauthorised to play 'Such a Whore (Stellular Remix)". Would you like to authorise it?'. If I sign in to authorise it (despite already being signed in) the error simply repeats.

I have tried deauthorising all devices and logging in again but this didn't help.

I get a similar message on my ipod and I cannot play the music file using another player.