You'll probably need to provide either a credit card or an iTunes prepaid card to pay off the balance you owe and be able to make further purchases. You can contact the iTunes Store Customer Service department using the form on their
Support page (select the category and subcategory closest to the issue you're reporting) and explain your situation to them, but there probably won't be anything they can do for you other than to tell you that you'll have to provide an alternate method of payment.
If iTunes wasn't going to accept, should have not went trough
If the card just wasn't accepted, the iTunes Store wouldn't have allowed you to enter it unless there was no way for the Store to distinguish it from a "normal" Visa credit or debit card. This is, unfortunately, sometimes the case with gift cards; they look like, but don't work like, standard credit cards. If that was indeed the case, then only when Apple tried to process the charge would they have discovered that the card would not be valid for the iTunes Store.
Message was edited by: Dave Sawyer