Did you create a new account or did you update your existing account with a new email address ? You can tap on the id in Settings > iTunes & App Stpre and sign out of it and you can then sign with the new/updated account.
If you did create a new account then your existing content will remain tied to your old account, so only that old account will be able to redownload that content and/or download updates to its apps. If you can't remember the password for it then you can try getting it reset via this page : http://iforgot.apple.com