Force Mac to detect displays?
This is really old but I found a solution that still works in OS X 10.11.2:
hit cmd + "brightness up" for display detection and cmd + "brightness down" to switch between mirror and extend display modes. It seems to be the brightness keys and not necessarily the F1 and F2 key.
Just press the option key while you're in Displays in System Preferences. You'll get a Detect displays button (bottom right).