Is that the Thunderbird master password? See https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-master-password?redirectlocale=en-US&redirectslug=master-password#thunderbird:win10:tb45 If it is only the mail password then you will need to address that with your mail provider as Thunderbird only stores the password you provide. If this is Thunderbird suddenly starts asking for a password. Then it is a completely different issue. Either your provider has changed settings, your anti virus is blocking Thunderbird or one of a multitude of other things.