I entered the code for my gift card but I never used it,now I'm trying to use it but it's no there and doesn't let me redeem the code again. What can I do?

An iTunes gift card can only be redeemed once, you can't enter and redeem its code again. Since you redeemed the card you haven't bought anything (including IAPs and auto-renewing subscriptions), and your account doesn't show a balance ? You've tried logging out and back into your account and seeing if it shows then ?

My account's balance shows at the top left of the store's page on my computer's iTunes, and by logging in to view my account via the Store > View Account menu option. On my iOS devices it shows at the bottom of, for example, the Featured tab in the App Store app under my account id (or you can view your account via Settings > iTunes & App Stores and it should show on the screen that you are taken to). If the balance is zero then it might not be shown.