Black screen after High Sierra update, MacBook Pro w/user-installed SSD
Mikey_S has answered the question on his/her own:


  a pal suggested shutting the lid, waiting 30 seconds, then re-opening
  the lid. That powered it down...somehow. Then I held down the power
  button till it restarted, and all is (now, thankfully) well. Hope this
  helps someone out.