Try resetting your cable box by unplugging the power cord for a minute or so.
If that doesn't help, call Comcast at the phone number on your bill or 1-800-Comcast and ask them to check the account setup and send a refresh signal to the box. If they can't fix the problem during the phone call have them send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have their Service Protection Plan (http://www.comcast.com/spp/, about $4/mo). If the trouble is due to a faulty Comcast cable box/DVR or anything outside, you shouldn't be charged.