Generally speaking the photographer, owns the copyright, however its not always that simple, they may have been made to sign a contract, and the festival may own the copyright, I'm speaking from experience here as I've been reviewing and shooting bands recently in another role, and there have been occasions where Ive had a contract thrust into my hands before i'm allowed into 'the pit'.
Sometimes its easiest to walk away as others have suggested, but it may be worth approaching the festival organisers and asking why your pic was chosen as you're not on this years bill, it can go either way but they may have an alternative they can use. Its got to be worth a shout and pointing out you're not on the bill, may just swing it.