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.