The router reset button is located on the back of the router. The reset button is small and recessed and is normally a different color than the rest of the gateway's housing.
For example, if the gateway is black, the button is likely orange or yellow. This makes it easier to identify.
Using a paper clip or pen, gently depress the reset button and hold it until the lights on the front of the modem turn off. Once the lights turn off, release the button.
After several seconds, the lights on your router will turn back on. It may take several minutes for your wireless network to reappear. Afterwards, be sure to change your Comcast Xfinity wifi password to something more secure or memorable.