I inadvertently hit CTRL-ALT-F8 and my secondary monitor went blank and all icons were moved to the primary monitor.
I researched this issue but I'm wary of even the one solution which sounded reasonable . . it said
1) Go to Start menu, search for services.msc.
2 )Find the "Service for Intel® HD Graphics Control Panel" service.
3) Disable it (open it's properties, stop it AND make sure to set its' startup type to "Disabled")
4) Open the task manager and kill all processes with names that start with "igfx".
I'd much rather get some advice from SAF.
BTW I did a cold boot, no effect, and Control Panel > Display > Change Display Settings shows both monitors under Display but when I click "Detect" it says "Another display not detected."