Try rebooting your box. Turn it off, unplug from power for about 10 seconds, then plug it back in. Wait about 2-3 minutes before turning it back on.
It's possible it lost it's authorization. If that doesn't fix it, you may need to call Comcast to verify your account.
Thank you for your reply. I rebooted the box, but regrettably nothing changed.
Moreover, when I press the center button (OK button), it doesn't work.
Should I call Xfinity or visit Xfinity shop?