First, make sure you're logged into the iTunes Store using the account ID and password you set up, or used, when you redeemed your card/gift. If you are, try signing out of the account and back in.
If you're sure you are using the correct account and signing out and back in doesn't help, go to the Purchase History:
http://support.apple.com/kb/HT2727
and review your purchases to make sure that you should indeed have a balance left. iTunes doesn't always refresh the balance it shows, so it's possible you spent the money and it just wasn't being reflected in the display. This display error can usually be corrected by logging out of the iTunes Store and then back in.
If you confirm that you should have balance left but the iTunes Store isn't showing it correctly, go here:
iTunes Support
and follow the instructions to report the issue to the iTunes Store.
Regards.