DIAGNOSIS The monitor is powered but not receiving a valid video signal from your source. Most likely, the input source on the monitor is set to the wrong port (e.g., DisplayPort when you're using HDMI) or the cable is loose.
FIX First, press the monitor's physical menu button to open the OSD. Navigate to 'Input Source' and select the correct port (HDMI 1 or HDMI 2). If that's already correct, power everything down. Unplug and firmly reseat the HDMI cable at both the monitor and PC ends. Power the PC on first, then the monitor.
IF THAT DIDN'T WORK Try a different HDMI cable you know works. Also, test the monitor with a different source (like a game console or laptop) to isolate whether the issue is with the PC's graphics output.
HEADS UP If you're using a GPU, ensure the HDMI cable is plugged into the graphics card ports on the back of your PC, not the motherboard's onboard video ports.