Press the Reset button and hold it in for a few seconds. This will clear all of your settings on the gateway and reset the user name and password to the factory defaults, which are:

User Name: comcast
Password: 1234

Please note that pressing and holding the Reset button will erase all your settings and affect the Internet connection of any connected computers on your gateway. You will need to reboot all connected computers to re-establish Internet connectivity and setup any features on the gateway you previously configured. Pressing the Reset button should only be done after you have exhausted all other troubleshooting options.