If you’re connecting your Mac to an  external monitor  using a USB-C cable . You should ensure the connectors on both your Mac
and monitor are completely inserted. You may not be able to recognize your
monitor even if there is a slight disconnection. 

If you are using
adapters to connect monitors that don’t use USB-C, make sure they are securely
connected to one another and your mac. There are particular issues with HDMI to
USB-C adapters that can prevent Macs from recognizing  external monitors .
Make sure your USB-C cable is connected to a DVI adapter. Furthermore, HDMI
cables for monitors will almost certainly cause your Mac not to recognize HDMI
external displays and are a deal breaker for the Mac. 

Learn more here: https://www.soldrit.com/blog/how-to-fix-multiple-monitors-having-issues-on-mac/