DIAGNOSIS: The monitor isn't detecting an active video signal from your PC on the input it's set to. This is almost always a cable or port issue, not the monitor itself failing.
FIX: First, power cycle everything: turn off the monitor and PC, unplug the HDMI cable from both ends, wait 30 seconds, and plug everything back in firmly. Turn the PC on first, wait for it to fully boot, then turn the monitor on. If it persists, try a different HDMI port on your graphics card and make sure the monitor's input source (using the joystick button on the back) is set to the correct HDMI port you're using.
IF THAT DIDN'T WORK: The HDMI cable might be the culprit, especially if it's an old or very cheap one. Swap it for a known-good, high-speed HDMI cable that's certified for 4K. Also, try plugging the monitor into a different source, like a game console, to rule out a PC graphics driver issue.
HEADS UP: If you're using a GPU with both HDMI and DisplayPort, sometimes the PC will default output to DisplayPort. You might need to go into your Windows/Nvidia/AMD display settings to detect the monitor or force the HDMI output to be active.