Did you enter any Billing Address details when changing to US Store? If so then you shoud check it and change it back to Hungarian address, aswell check if you have anything at Payment Information in iTunes if so delete it or change, add a Hungarian card if you have posibility.
Else, as Niel wrote you will need to contact iTunes Support, but as an extra information issues with EU people shoud go to: http://www.apple.com/emea/support/itunes/contact.html and there you can select that you have issue in US Store (Support will answer you in max. 72 hours).
Hope it helps,