DIAGNOSIS The monitor is powered on but can't see the signal from your source. This is almost always a handshake issue or the monitor is looking on the wrong input.
FIX First, use the monitor's physical joystick button on the back right to open the OSD. Navigate to the Input Source menu and manually select the correct input (HDMI 1 or HDMI 2). If it's already correct, power cycle everything: turn off the monitor, unplug the HDMI cable from both ends, restart your laptop, then reconnect everything and turn the monitor on last.
IF THAT DIDN'T WORK Try a different HDMI cable, preferably a certified High-Speed one. The monitor's HDMI ports are version 2.0, and a subpar cable can fail at 4K. Also, check your laptop's display settings to ensure it's set to duplicate or extend, not just outputting to its internal screen.
HEADS UP If you're using the USB-C port for video, it takes priority. Having a USB-C cable connected can sometimes cause the HDMI ports to be ignored until you manually switch back.