I’ve been having issues getting my 2011 MBP to recognize my second monitor ever since the Yosemite update. Even the Detect Displays trick didn’t help. But, finally, I found one thing that worked for me every time: Command+F2 typed on the laptop keyboard (doesn’t work on external keyboard FYI) runs an apparently superior “Detect Displays” command, and like magic the second screen is recognized and starts working again.
Found this trick on the end of this 6-year-old forum:
After this worked flawlessly for a week, it stopped working altogether. Now, I can’t get the monitor to be recognized no matter what voodoo I try.