Diagnosis
The TV isn't recognizing the HDMI input from your cable box, usually because the HDMI signal format setting is wrong for the box's output.
How to Fix It
Change the HDMI signal format on the TV's input. Press the 'Input' button on your remote, highlight the HDMI port your cable box is using, press the 'Options' button (three dots), select 'HDMI Signal Format', and switch it from 'Enhanced format' to 'Standard format' (or vice versa). Reboot both the TV and the cable box.
If That Didn't Work
The HDMI cable itself might be the issue. Try a different, known-good HDMI cable, preferably a certified High-Speed one, and plug it into a different HDMI port on the TV to rule out a port problem.
Heads Up
If you have a 4K/HDR device like a PlayStation 5, you'll want that specific HDMI port set to 'Enhanced format' for full resolution, so plan your port assignments accordingly.