Macbook Pro suddenly stopped detecting external monitor

The Apple support helped me!

The solution was:

Disconnect monitor

go to: /library/preferences

find & delete files:

com.apple.windowserver.plist

com.apple.windowserver.displays.plist (if available)

Go -> option -> user library -> preferences -> by host

find & delete these files

com.apple.windowserver. + alpha numeric string

com.apple.windowserver.displays.plist + alpha numeric string (if available)

Empty bin

restart

reconnect monitor

In case of doubt call them, really helpful! :)

The "approved method" that does NOT require you to fiddle in the guts of macOS is to invoke Safe Mode.

Safe Mode does a number of different things. On older Intel Macs, hold shift at startup, but have your userid and password at the ready.

A parade of unusual things happens.

• Your Mac loads just enough of the kernel to do a disk check. Then it proceeds to do a disk check. This can take an extra about five minutes.

• your userid and password are required, even if you normally auto-login. So have them handy.

• Your Mac adds ONLY a minimal set of Apple-Only extensions, Not including graphics acceleration extensions. Screen updates will therefore be wonky and slow, but it ultimately should be correct.

• Your Mac assumes defaults for as many settings as possible, including screen resolution. This is the key for re-setting the screen, but there is a little more to it: Resolution is likely to be lower and settings ordinary. Use this as a starting point to customize settings to your liking.

Any changes you make in Safe Mode will "stick" in regular mode after you restart.

• after restart in normal mode, your Mac will take slightly longer to start up [once] because it rebuilds some system caches.

this article explains how to start in Safe mode:

Start up your Mac in safe mode – Apple Support (UK)

ASUS display VY229HE appears to be a display capable of 1920 by 1080 with

HDMI input.

To use with a modern Mac at modest resolutions you should be using a Certified cable labeled PREMIUM. if used with a display close to 4K resolutions, the cable should be labeled ULTRA and is backward-compatible.

I have the same problem on my m4... :/