I work with my MacBook Air connected to a Apple Thunderbolt Display. Sometimes, I’ll find that my Thunderbolt Display shows my screen, but my MacBook Air remains black. Boo.
In OS X Lion (and possibly Snow Leopard), the Display preferences pane had a Detect Displays button which fixed this for me.
That button doesn’t appear in Mountain Lion. Or does it?
If the detect displays option is not available, hold down the Option key while you are in the Display pane.
Oh. Thank you, HT5369.
All Narfed Up 