Try a quick 2 second press of the recessed reset button on the back of the gateway using a pentip, give it time to come back up, see how you go.
Otherwise see the troubleshooting here:
Hi, llynnll - I can check if there are any issues with the RF signal levels or equipment. Please send me a private message and include your full name, the account holder's name and address as it's listed on the billing statement, a phone or an account number. Click on my name ComcastElla, then click Private Message Me.