i connected my laptop to my lg 27ul500-w with hdmi but the screen is black. the laptop says it's connected. help?

This is usually an input source or resolution mismatch. Let's get it sorted.

1. First, press the monitor's joystick button to open the On-Screen Menu.
2. Navigate to the input selection. Your monitor has two HDMI ports and one DisplayPort. Make sure you've selected the correct HDMI port (HDMI 1 or HDMI 2) that your cable is plugged into.
3. On your laptop, go to display settings and ensure the resolution is set to 3840 x 2160 and the refresh rate is 60Hz for the external display. That's the max resolution for HDMI on this model.
4. Try a different HDMI cable if you have one handy.

IF THAT DIDN'T WORK: Try the other HDMI port on the monitor. Also, check if your laptop's graphics driver needs an update, as that can cause handshake issues.