Q: itunes prepaid card

Was the gift card issued in the same country as your account is based ? They are only valid in their country of issue. If it was and this page doesn't help then you will need to try contacting iTunes support (you will need to give them the serial number and activation code from the card) : 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

Did you enter the code on itunes? (on the right, there is "use the code" window)?

There are instructions on this page for how to redeem a card : http://support.apple.com/kb/HT1574

But was the card issued in the same country as you and your iTunes account are in ? If not then you won't be able to use it. If it was issued there and you are getting an 'invalid' message and this page doesn't help then you will need to try contacting iTunes support as described.

If your account is based in the same country as the card then have you tried contacting iTunes support ?  If your account is not based in the same country as the card then do you know anybody in that country that you could sell or give the card to ?

And your account is also based in that country ? You can contact iTunes support as I described in my first post