I just ran into this, will post the quick fix in case anyone else runs into this...   
Of course, use Notepad++   
In the options.lua file, find the ["graphics"] section.   
Find the ["multiMonitorSetup"] entry.     
You'll probably recognize the entry in quotes on this line to what you selected in options.  To return this to a single screen, change the text in the quotes to "1camera"  (That's a number 1, not an L)  Save, restart DCS.