Note - This applies if end user forgotten the valid-user and password combination for mod_auth_basic module.
Edit the .htaccess file in your favorite text editor and comment out the authorization module entries and "save, exit" the /var/www/html/.htaccess as following:
nano /var/www/html/.htaccess
#AuthUserFile /var/www/.htpasswd
#AuthType Basic
#AuthName "Restricted"
#Require valid-usersave & exit and refresh your web browser, access to the http://your.pbx.ip/admin/config.php should be granted without username/password.