DIAGNOSIS The monitor is receiving power and a signal, but the backlight or panel isn't activating. This is almost always a handshake failure with the input source or cable.
FIX Cycle the input source. Press the monitor's joystick button to open the OSD, navigate to 'Input Select', and manually switch to the correct port (DisplayPort or HDMI). If that doesn't work, power everything down and reseat both ends of the video cable.
IF THAT DIDN'T WORK The monitor's 'Deep Sleep' mode can sometimes get stuck. Unplug the monitor from the wall outlet for 2 full minutes, then plug it back in and power it on.
HEADS UP If you get a picture after reseating the cable, the cable itself is the likely culprit. This monitor's high refresh rate at 1440p demands a high-quality HDMI 2.0 or DisplayPort 1.2 cable; a cheap or old cable can cause this exact symptom.