Self Help for connectivity outage
In addition to the general connectivity troubleshooting tips already stickied in this forum there are 3 other tips to be aware of:
1) Modem reset. If your Comcast modem IS NOT running static IP addresses than try a hard reset using the reset button, Make sure to hold the button down a full 30 seconds. Sometimes people don't hold it down long enough. This is mentioned already in this forum but not how to do it.
2) Check the AC adapter status on the modem. You will need a regular voltmeter and 2 pins. VERY CAREFULLY press the pins into the insulation of the DC power cable coming from the AC adapter (not close to each other you don't want to short them) just enough so the tips of the pins contact the wires inside then measure the voltage on the pins while the modem is powered up. Sometimes the adapters fail under load - an unloaded voltage test will NOT show this
3) DO a reverse traceroute test. To do this you will need a second Internet connection (maybe you can tether to your phone) Access the cable modem using the customer access interface. Get the cable modems public IP (wan address) Now, switch to the working connection, and access a reverse traceroute site from this list http://www.traceroute.org/ then initiate a traceroute to your public IP. The trace should complete at your modem's IP if it does not then it should complete at a comcast router in your city (or close to it) Most of Comcast's routers have pretty descriptive names so it should be obvious if there is a gross discrepancy.
Note that if you have a completely down circuit your modem will NOT have a valid WAN IP address and a reverse trace will be meaningless.
lastly, in any outage situation it is most important to contact technical support as soon as you can. If you get a recorded message saying there is an outage in your area and your service outage just happened then give it a few hours and see if it gets fixed but don't let it go longer than that - if it's still out, call back.