I have a retina MacBook Pro running Mountain Lion with a 23" external display attached through the Thunderbolt port (DVI/Display Port adapter). I have the monitor configured as an extension to my internal display (arranged just above as it actually is placed on my desk).
The arrangement works very well. However, sometimes (~30-40% of the time) the external display remains blank if the MacBook Pro screens have timed out or the MacBook Pro has gone to sleep and I then wake the MacBook Pro (by pressing a key). The MacBook Pro still thinks that the external display is connected as the screen remains in extended mode and apps are also there (though invisible, if that's where they were placed before the screen/MacBook Pro went to sleep). If I disconnect/reconnect the display port (Thunderbolt), it does go to internal only, but I cannot get the external display back unless I restart the Mac. Any ideas?
Best Answer
To get the display back without rebooting, I've had success doing this:
To avoid the issue altogether, I've gotten into the habit of disconnecting the display before I leave the Mac.