I would do what gaborm suggested. Unplug the unit from the power outlet while it is still on. Wait 1 minute, then plug it back in and turn it on. It's important you unplug the unit while it is powered on, to preform a hard reset.
After you turn it, you may get a message that says "One Moment Please.. this channel will be available shortly". This is normal, as it is receiving data again (since it just went through a hard reset.
After about 20-30 minutes, everything should be working.
If it still doesn't work, call Comcast and ask them to "reinitlize" your box. If they say they are sending a hit (or a signal), tell them that won't do anything for you really.
If you do have to swap out the box, you will lose all your recorded programs, and settings, etc.--
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