I have a problem where occassionally I will reconnect a 15" Macbook Pro to my 27" Cinema Display and it stays black and seems unresponsive to the power key on the right side of the ACD. Sometimes attaching a different MBP seems to get it to wake up, but not always.
But I just found something that cleared this recurring pesky problem for me immediately.
My symptoms: I'd untether my Macbook Pro (MBP) from my Apple Cinema Display (ACD) and use the MBP standalone for a while. Later I would hook it up again to my Apple Cinema Display (ACD).
Normally, it would go back into 2 monitors mode and the ACD would immediately show the 2nd screen. However, occassionally, the ACD remains black even though the Displays tool bar menu indicates it sees the ACD display as well as the builtin display -- however the ACD remains black -- and insensitive to turning on and off the ACD Power button on the right of the display.
My Solution: I go to System Preferences, select Displays and then go to Arrangement and enable the Mirror Displays mode, so I can see the ACD panel.
Now that I can see the ACD panel, I go to Options. Under "Display Power Button" I find that the menu item selected is "Turns the display on and off" is checked. That might be what I would like, but it isn't what is happening! So I click "Does nothing" to disable the button. Suddenly, my ACD shows the mirrored display in all its glory!
Now I go back to Arrangement, and turn off Mirroring again. Now my display remains on Whenever it is connected to a MBP.
I hope this helps others. I suspect that something confuses the internal software of the ACD so that it is ignoring the Display Power button (as is the case with "Does nothing"), but it thinks it is in "Turns the display on and off" and in the "off" mode. But since the Display Power button is a toggle and it is ignoring it, it remains stuck in the off mode.
Anyway, clicking "Does nothing" helped clear the situation for me. I hope it works for you.