Had the same thing.
The PC is often left on,Came back to desk to find Windows10 had been installed.
Started misbehaving straight away-Disabled 3rd monitor
as apparently the graphics card had driver issues and needed to be reinstalled.
Which wouldnt be particularly strange if it wasnt for the fact that the system uses two identical cards-both using the same Nvidia drivers.
After reinstalling the drivers-the 3rd screen
was then recognized,but would "white screen" any time anything than than the desktop
Pushed that aside,to find that some software
i have also didn't appreciate the "upgrade" to Win10..
Very easy decision to simply hit the "go back to windows 7" button...
Really wish they wouldn't do stuff like this... Annoying to say the least.