Diagnosis
The monitor is receiving power but no active video data from your source. The most likely cause is the input source setting on the monitor not matching the port you're using.
How to Fix It
First, verify the cable connection at both the monitor and PC. Then, press the physical menu button on the monitor to wake the OSD. Use the joystick control to navigate to 'Input Select'. Confirm it is set to 'HDMI 1' or 'HDMI 2', matching the port your cable uses.
If That Didn't Work
Test the HDMI cable with another device. If the cable is good, the issue is with the PC's output. On your PC, access display settings and force a detection of the second display. Try a different video output port on your graphics card.
Heads Up
The monitor will default to the last used input. If you switch sources frequently, you will need to manually select the correct input each time using the OSD menu.