Sounds like I'm having the same type of problem. It happened at least a month ago. On xfinitytv.comcast.net/tvlistings, the grid starts at channel 14 or sometimes 17, but should start at channel 1. I can't even control my DVR from here because it doesn't show Channel 1.
I have the the correct zip code entered. When I click on change location, the pop up window shows the correct channel listings; however, the grid doesn't change, regardless of the service area that I select.
Have contacted comcast support, but no help.