I’ve read a few reports on this today. It’s being claimed by Prevx that one of Microsoft’s recent security updates causes issues on machines. The machine boots up OK, but you get nothing but a black screen when you log in. The have posted a “fix” (it’s not something MS has authorised). Note that this is an issue that can be caused by a lot of things. I’ve personally seen it a few times over the years.
Microsoft are reported to be investigating.