You should talk to your email provider about password reset options.

Christ1, thanks for the reply... I do have my own domain which use just for e-mails... So the smtp settings are my own, but I can´t remember the password

I suppose you did receive email server settings from your hosting provider? Then you should probably check with your hosting provider about a password reset. Did you tell Thunderbird to remember the password by any chance? Then you may be able to recover it from the Thunderbird 'Saved Passwords' underneath Settings. However, if that was the case then you shouldn't have got a password prompt in the first place.