To restart your modem, leave your computer on and unplug the power to the modem (only the modem) for approximately one minute. Plug the modem back in and try to reconnect.
If the connection still fails, do the following: With the computer on and modem power on, press the reset button on the back of the modem (it is a small hole in which you will need to insert something equivalent to the end of a paper clip) for approximately one minute. When you let go you should see the top three lights lit up solid green. If the connection still fails, contact a technical representative at 1-888-Comcast for further assistance.