External monitor not working when in closed clamshell mode I have a mid-2015, 15 inch MacBook Pro, connected to a Dell P2715Q with a Display Port to mini-DP cable. With the lid open, I get both displays working.

I used to be able to close the lid and it would switch to just using the external monitor, but I have changed something in the settings so that when it is closed, the display is black.

I am using SwitchResX, to change the resolution, and I think I changed the colour from Millions to Billions, which broke it.  I now can't change it back as it is only broken when the lid is closed.  I want to have just the single monitor.  How can I change the settings back to normal so that it works properly?

The MacBook Pro is plugged in to mains power, and is connected a USB Keyboard and Bluetooth Mouse.

I use the exact same setup at work, with identical Keyboard and Mouse, but a different monitor, and that works fine.  It appears that the configuration for the monitor in open clamshell mode is different to that in closed clamshell mode, and I'm trying to find out how to re-configure the closed clamshell mode when it does not display anything with it closed.
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.