For the last week or two my 2nd monitor will not come on after a straight boot. However, I have no problems if I do a restart.
I've updated the graphic card (FirePro W1700) to 15.301.2601.1002 (7/26/2016).
I have also done a system restore in Windows 10 to before the anniversary update, but this has not helped.
During startup from a straight boot, my system recognises both monitors and the fact that it's extended over 2 monitors, it's just that monitor 2 will not come on. I have to type in my password 'blind' over a black screen to get into windows and then do a restart.