Diagnosis
Your monitor is powered on and working, but it's not receiving a video signal from your computer. The most common cause is the HDMI cable isn't seated properly or you've plugged it into the wrong port on your PC.
How to Fix It
First, power everything off. Unplug the HDMI cable from both the monitor and your computer. Look at the back of your PC—if you have a separate graphics card (usually lower down), plug the HDMI cable into that port, not the one on the motherboard. Push it in until it clicks. Do the same on the monitor. Turn your PC on first, wait a minute, then turn the monitor on.
If That Didn't Work
The monitor might be stuck on a different input. Press the joystick button on the bottom of the monitor to bring up the menu. Navigate to 'Source' and make sure it's set to 'HDMI', not 'DisplayPort' or anything else.
Heads Up
If you're using a laptop, sometimes you need to manually tell it to output to the external monitor. Try pressing the Windows key + P and selecting 'Duplicate' or 'Second screen only'.