If Thunderbird was missing then it was uninstalled by whomever did the computer updates. The password that is being requested is the same password you would use to access your webmail account via a browser. Passwords can only be set up/modified via the webmail account. When attempting to logon to webmail, if you do not know password, there should be a 'forgotten password' or similar link. You would need to follow the instructions to gain access, then reset password. Then use that same password in Thunderbird, so Thunderbird can access the server.