Diagnosis
This is almost always a handshake issue between the monitor and your graphics card. The monitor is powered on but isn't receiving a readable video signal from the source you've selected.
How to Fix It
First, let's make sure we're talking to the right port. 1) Check which input port you've plugged the cable into on the monitor (likely DisplayPort or HDMI). 2) Press the joystick button on the back of the monitor to open the OSD menu. 3) Navigate to the Input Select option and make sure it's set to the exact port you're using. 4) Power everything down. Turn off your PC and monitor. 5) Reseat both ends of your video cable firmly. 6) Turn on the monitor first, wait a few seconds, then turn on your PC.
If That Didn't Work
The cable itself might be the culprit, especially if it's a cheap or very long HDMI cable not rated for 1440p high refresh rates. Try a different, high-quality cable. Also, try plugging the monitor into a different video output on your PC if you have one, to rule out a bad port on your graphics card.
Heads Up
If you're using a DisplayPort cable, it has a latch. Don't just pull it straight out—you need to press the release button on the connector first to avoid damaging the port on your monitor or graphics card.