It happened to me once before: After upgrading an old laptop to Windows 10 free, I ran into this problem. Must log in to Windows blindly: click and entering password not seeing anything. If lucky, the desktop would show. Dual monitor worked. After searching the internet I found nothing relevant. I leave that laptop.
Today it happened to me again on my working laptop – when I was playing with battery options.
Solution to my problem:
Somehow the rightness was set to the left end, on 0%. After sliding right to the 100%, the primary monitor was showing. It was there all the time. Fortunately, the external second monitor was working, so I was able to find the solution.