Did you owe iTunes any money and/or have any pre-orders or auto-renewing subscriptions that were due ? You can view the purchase history on your account via your computer's iTunes : log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases.
If not then have you tried logging out and back into your iTunes account on the device/computer that you are using and seeing if the balance then updates ? If it doesn't then you could try contacting iTunes Support and see if they can help with where it went : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption