Are you sure you're using the right login information?
An old account could have been registered to an email without the email being the login information for the account.
Using email to login is for accounts made after they implemented email as user login. Older accounts still log in with their original username.
Old names may have been reset, in which case you would log in to your account with a bunch of numbers instead of your old name.
You can freely choose a new name, assuming the name you want is available.