No signal on external monitor after High Sierra upgrade

Solution 1:

I just had the same issue. Found this link to upgrade the display port drivers, it was working again after a installing and restarting: http://www.displaylink.com/downloads/macos

Solution 2:

I fixed the problem--by using an Apple TV device to create my extended desktop. It was super easy and it remembered to go back to the extended desktop setting when I turned it back on later.

Solution 3:

Couldn't connect external monitor, drove me crazy. Solution: Used a VGA cable with a USB-C adapter and it works. HDMI and everything else won't. VGA cable + buy a VGA-USB-c adapter = saving a lot of money.