IDid you create a new account or update your existing one ? Content is tied to the account that originally bought/downloaded it, so if you created a new account then you need to continue to use the old account to download updates to its apps. If you updated your existing account then what account shows at the bottom of, for example, the Featured tab in the App Store app, the old or the udpated version ?
If you did create a new account and your old account is disabled then you might be able to re-enable it via this page : http://appleid.apple.com, then 'reset your password'
Or you might need to contact Apple : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page.