my LG 27UL500-W monitor won't turn on, the power button does nothing. what's wrong?

Power delivery is the issue. Start with the power adapter and its connections.

1. Check that the external power adapter is firmly plugged into the monitor's DC input port and into a live wall outlet.
2. Verify the power adapter's AC input range is 100-240Vac, 50/60Hz. Ensure your outlet voltage is within this range.
3. Try a different power cable between the adapter and the wall, if possible.
4. Unplug the monitor from power for one full minute, then reconnect and try the power button again.

IF THAT DIDN'T WORK: The problem could be with the video source or cable. Connect the monitor to a different source device using a known-good HDMI or DisplayPort cable. Ensure the source device is powered on. If the monitor receives a signal, it may power on.

HEADS UP: In Sleep Mode, power consumption is less than 0.5W. If the monitor is in sleep, pressing any button or moving the mouse on the connected PC should wake it. If it doesn't, proceed with the power check.