It is your email provider's server asking for the password. Thunderbird is just passing on the password you typed, or the password you told Thunderbird to remember. If the latter one doesn't work anymore try to login to your account via webmail. If your password somehow got lost, you'll have to check with your email provider about password reset options. Typically the password for sending and receiving messages is the same. So if you can still receive messages via Thunderbird, chances are that Thunderbird has remembered your password, and you can look it up in your settings.