Are you using a Router?
Are you just using cable modem connected to a PC?
Have you updated the firmware on the router?
Have you updated the drivers for the network card on your desktop or laptop?
Do a power on and off on the cable modem then on the router if you're using it.
If you just have the cable modem contact comcast has let them remote into your cable modem to see if their is a problem with it. If they find a problem you can, get one from them or save some money and go to one of the electronic stores and buy a cable modem. If you buy one on your own you have to tell comcast the mac address on the unit so they can join you to their network otherwise you won't be able to get onto the internet.