I'm not sure why losing an email account on your iPad would affect your online banking, but:
Go to Settings > Mail, Contacts, Calendars and check out the accounts. If your account is still there, select and Delete it. If you have any calendars or contacts synced with that account choose the delete option when prompted. This only deletes the info on the iPad.
Re-add the account. Go down to the Mail settings and reselect it as the default account.