Make sure your router is powered on and running normally.
On the back of your router is a small recessed button.
Use a paper clip or pen to depress the button for about 15 seconds until the router automatically resets itself and powers itself back on.
Your FiOS router admin username and password will now be reset, along with any other settings you've customized -- including the WiFi network name and password.
You can now log into your FiOS router to change your admin and network passwords.