First just try unplugging your cable box, waiting about 5-10 minutes, then plugging back in, and wait about half an hour. If you're still having the same problem, give Comcast a call so they can "zap" your cable box again, I did it myself recently, just follow these steps once prompted:

(Note: you have to wait for the recording to finish, you can't cut her off )

1. Press 2 for "All other inquiries"

2. Press 1 for "Trouble with service"

3. Press 1 for "Cable television"

4. Enter your Comcast account #, or the last 4 digits of your Social Security Number

5. Press 2 for "For total service interruption"

6. Press 1 for "For blue, blank screen or error code"

7. It'll ask you to make sure everything is plugged in and turned on... then press 2 when its done talking

8. Press 1 for a Signal Refresh (the "Zap")

In short, here's the above again: 2, 1, 1, (acct or ss #), 2, 1, 2, 1. It'll say that it can take up to 20 minutes, although it can take as little as 1 minute. It'll also tell you to wait 20 minutes and if it didn't work, unplug your cable box from the outlet, wait 30 seconds, and try it again.

Hope that helps!