Mac not detecting external display suddenly

Given the HDMI cable, Mac’s port, and the LG display all work fine independently with other setups, the issue seems to lie somewhere in how your Mac and the LG display are handshaking with each other specifically. The fact that Detect Displays does nothing and you've tested refresh rate/resolution options rules out common software misconfigurations. This could be due to EDID caching issues or a hiccup in how macOS is identifying the display.

I suggest you start by power cycling 'everything:'

• Shut down your Mac completely.
• Disconnect the HDMI cable and power cord from the LG display.
• Wait about 30 seconds, then reconnect the display's power.
• Boot your Mac and reconnect the HDMI.

If that doesn't resolve the issue, boot up your Mac in Safe mode to see if it can help:

• Use safe mode on your Mac - Apple Support

Create a new user account and log in. Sometimes user-specific preferences or corrupted display profiles can cause these issues.

f you have a USB-C to HDMI or direct USB-C to DisplayPort cable, see if the Mac sees the monitor through that. This can confirm if the macOS HDMI output stack is the culprit.

Finally, If none of the above resolves this issue, delete the EDID Cache:

• Go to `~/Library/Preferences/ByHost/` and look for files that include `com.apple.windowserver` and a long UUID. Trash those and reboot.