Here are the tricks as I had a major outage...
1. Reboot the router and cable modem and if you have an Apple router, use the Airport Utility to see if it is up and what it is complaining about. If the internet is down, it will complain or say there is no DHCP server.
2. Check to see if you can see your router, it has the magic address http://192.168.1.100 and that should give you a web page. If you can see it, you want to see if the 54dBM upstream is the key parameter
|Freq/Power:||609.000 MHz||12 dBmV|
|Freq/Power:||37.000 MHz||54 dBmV|
|Channel Type:||DOCSIS 2.0 (ATDMA)|
|Symbol Rate:||5120 kSym/sec|
3. If you don't see this, check to see if your cable TV is working, if not, then you have a big outage and call 1888COMCAST
4. Ask them if they can see the router and then they can hard boot it.