External monitor not working when in closed clamshell mode
I had the same problem:
Changed refresh rate to 144 Hz while in clamshell mode --&gt; Blackscreen.
But everything worked fine when I opened the Laptop. The problem is that the settings can only be changed back if in clamshell mode because they are stored separately. But how to change them back if the screen is black?
Solution: Team Viewer.
Connect your laptop to the monitor. Get an iPad or second PC and connect via team Viewer. Close Laptop. Now you should be able to see the settings for clamshell mode. Just change them back and everything works fine again.