I had this issue after replacing the screen on my late 2013 Macbook Pro. I opened up the machine again and reseated all the monitor related ribbons/cables that I touched during the replacement and it now works!
+
+
Before you open up the casing again, try closing the lid (to sleep) and re-opening. Weirdly this can work.
I had this issue after replacing the screen on my late 2013 Macbook Pro. I opened up the machine again and reseated all the monitor related ribbons/cables that I touched during the replacement and it now works!