This seems to happen most often to Sony "smart" televisions. It's been an issue for more than a year now. You could try (using the tv remote) switch the source to another port. Then back to the correct port for HDMI cable. If that does not work, then unplug the X1 box for over a minute. Plug it back in to allow reset.
This usually works but is a real pain in wasted time.
The same issue is with Toshiba TV... Xfinity box has a defective hdmi connection and they will not take ownership and fix it.
If the HDMI port on the X1 box is defective, the only fix is to go to the nearest Xfinity Store and do a free swap.