Your email account has a password. It's your responsibility to know what it is. You may need to work with your email provider to re-set it. In Thunderbird, '''Tools|Options|Security|Passwords→Saved Passwords''' will tell you what passwords (if any) have been stored. Login failures like these are often due to temporary problems with the connection, the server being offline or busy and so on. Often "it just works" when you try again later.