Think you can set default authentication via config.php file.
But then whatever login you attempt to use needs to be manual and you have to use the 'normal' login screen's login dialog boxes.
Usually, the first user created when site was installed was/is the initial admin user. Default username for that account was 'admin' ... unless changed.
So use command line query of moodle DB for the site of mdl_user table for all accounts whose auth column is manual.
If 'admin' user exist (id 2), but you do not know the password, change the password via command line.
You will be prompted for which manual user account, then for the new password.
Since you might not remember the password requirements ... pretend defaults are in affect .. 1 Cap, 1 none alpha numeric character and at least 8 characters total if I remember correctly.
'spirit of sharing', Ken