redeem card been used

Did the card still have the silver strip on it when you bought it, and have you tried going back to the store that you bought it from ?

You could try contacting iTunes Support (you will probably need to give them images of the front and back of the card, and possibly its receipt), but they are possibly unlikely to be able to help : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes

If they can't then try contacting the issuer of the card that you used to buy the gift card.