Occasionally fall asleep watching a streaming video, or a QuickTime movie seems to be the same problem, monitor has power saving so turns off when turning back on its a black screen with a cursor, can usually pull the cursor to the bottom corner for screen saver, has no effect, all keyboard combinations just been, only method I'm aware of is the on/off button on the iMac itself.
OS X Mountain Lion (10.8)
If you have made Quicktime fullscreen, you can reverse this by clicking in the upper right corner of that black screen. You will see a blue box with opposing diagonal arrows. Click that and Quicktime will revert back to a normal sized window.
If you are using the VLC movie player, they thoughtfully placed the full-screen toggle button on the player control bar.
Unfortunately there is nothing other than the black screen and a cursor, slides on the Magic Mouse does nothing, there is no menu bars from the movie players, no nothing blue shrink button, just blackness.
Press 'command + F' to exit full screen in both Quicktime and VLC. Use the 'esc' button to exit Frontrow.
Can't exit full screen after video.
