Sign Out of your Account... Close All Open Apps... Perform a Reset...
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.
i had same problem
appstore lets you download apps assuming you have enough to cover it in your account
if you dont they cant bill you or take money from account,untill there is enough in the account to cover the bill
you wont be able to download apps,updates from app store untill you have enough to cover the bill
hubby transfered funds to my account
retried appstore,entered details
i can now access app store,apps,updates
hope this helps