A likely reason for not being able to log in is due to having multiple accounts set up with the same email address. You will need to log in using your username instead.
If you cannot remember your username then please email email@example.com and request your usernames to be sent to you.
If you would like to be able to log in with your email address in future you will need to have all your duplicate accounts cancelled. Please email firstname.lastname@example.org and state the username(s) of the account(s) you would like cancelled.