Mac OS 10.13.5 - Dual monitor set up for MacBook with single USB-C
There is an issue with the DisplayLink Driver and the macOS update 10.13.4.

From the macOS page on DisplayLink's webpage:


  Installing macOS 10.13.4 will cause DisplayLink connected displays to
  go blank after the OS update when using DisplayLink driver v4.1.
  Extended displays continue to work as expected in macOS 10.13.3, and
  we can only recommend that you stay on macOS 10.13.3 or earlier if you
  require this feature and have not already updated. Functionality such
  as Ethernet and audio, where implemented, is unaffected.


You have to downgrade to 10.12.3 and the best way to do that is to revert to a Time Machine backup.  The second best way is to reinstall.  There's currently no way to roll back an update.