It could be parental protection. You didn't mention the type of connection between your Mom's cable box and TV. I'll assume it's HDMI. It could be just a "handshake" issue between your TV and cable box. You've already rebooted your cable box, so, yes, try rebooting your TV by unplugging it for a minute.
HDMI supports copy-right protection against piracy. Technically, it's called High-bandwidth Digital Content Protection (HDCP) and it's an authentication protocol. Your TV and cable box each have identification data and encryption keys stored on their chips. Before a cable box will send protected programming, it has to authenticate the data and keys on your TV. Somehow that step fails and it's probably what your Mom sees on her screen. I'm assuming the TV data and keys are corrupt.
It could be a faulty HDMI cable, faulty cable box, out-of-date firmware on the TV or a faulty TV. I'd try rebooting the TV before troubleshooting anything else. If the error messages persist, try updating the firmware on the TV. Thereafter, replace the cable box. Thereafter, replace the HDMI cable. What kind of TV is it?
It's a very old TV. My mom refuses to use a flat screen. It's always she's never touched it and she lives alone. Would you just unplug the TV to reboot (correct)? Otherwise I might get a new box. We are dealing with this multiple times a week. However, when this happens, it only happens on that TV not the other tiny ones she has.
Yes, its one with the large back....I'm at work and can't call Cox but I think I will ask about a new box. My husband doesn't think she will allow me to put a flat screen in there. Thanks for your input. Someone said something about the TV control. We have that at our house to prevent her from mixing things up more. She only has the Cox control at her apartment.