Diagnosis
The monitor is not receiving a stable video signal from the PC via the DisplayPort cable. This is most often a cable or handshake issue.
How to Fix It
Perform this sequence. One: Power cycle the monitor. Unplug it for 60 seconds. Two: Reseat both ends of the DisplayPort cable at the monitor and PC. Three: On the monitor, open the OSD Menu. Navigate to Display > DP 1.4. Toggle it OFF. Wait 10 seconds, then toggle it back ON. This resets the handshake.
If That Didn't Work
The cable is the prime suspect. Test with a certified VESA DisplayPort 1.4 cable. Also, try the HDMI port on the monitor with an HDMI cable as a diagnostic step.
Heads Up
If you have a discrete GPU, ensure the monitor cable is plugged into the GPU ports, not the motherboard's video ports. The motherboard ports are often disabled when a GPU is present.