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What make/model is it? Have you looked at the User Manual https://www.xfinity.com/support/articles/broadband-gateways-userguides?
One way would be to use a bent paperclip (or similar device), find the pinhole sized, recessed Reset button in the back and press it with the paperclip and hold it for 30 seconds, or until the lights all go out.
Another way would be to log onto the gateway's admin control panel at http://10.0.0.1 and go to Troubleshooting |Restore/Reboot Gateway and select Restore Factory Settings.