I have a 14" 2021 MacBook Pro with Sonoma 14.4.1 and an external LG Ultra HD monitor.
I use the external monitor to extend my MacBooks's display, this worked fine for a long time.
All of a sudden, the external monitor stays black when extending the display. When using the monitor as mirror, or even as main display, all is good.
I can even move the mouse pointer into the extended display and sometimes the extended display show the dock, the rest of the display remaining black.
I tried to use direct HDMI cable as well as a USB-C to HDMI adapter. Both worked fine in the past, and both work fine when mirroring or using the monitor as main display.
So cabling, the monitor and die MacBook HDMI and USB-C hardware can be ruled out.
When updating to Sonoma 14.4.1, I even saw that extending the display worked for a brief period.
Is there something I can do about that?