I have a Samsung S24C650 monitor and after upgrading my MacBook Pro (13-inch, Mid 2012 to High Sierra I cannot use the external monitor.
I have a hub that connects to the thunderbolt, and from the hub an hdmi cable that runs to the monitor.
The monitor works correctly, It is tested with a linux distro on another machine. The cable is fine (obviously) The hub is fine, tested with another Macbook 2015 model
The trouble started after upgrading to high sierra, initially the icons where not shown, computer failed to return from sleep (forcing restart) and last and most important the external monitor gets no signal.
When I plug the hub on the thunderbolt, Macbook's display changes (the computer does recognize that an external monitor has been plugged in.
I also have reFind install for dualbooting into Debian, when dualbooting into Debian the same thing occurs.
Any ideas welcome
Edit: Looks like this is an issue... https://discussions.apple.com/thread/8085619?start=0&tstart=0