Today I tried to press the Command-Tab shortcut for the Application Switcher but nothing happened. Another thing that wasn’t working was moving the mouse to the edge of the display to make the hidden Dock appear. The keyboard shortcut to display the Dock wasn’t working either. If this happens to you, the way to fix all of these problems is to restart the Dock. Open the Activity Monitor (it’s in the Utilities folder), select Dock in the list, and click the Quit Process button (or choose View > Quit Process). In the confirmation dialog box that appears, click Quit. The Dock is one of those processes that restarts itself if it’s quit, so that’s all you should have to do. The Dock and Application Switcher should both work now.
Not everyone is aware that the Dock hosts many processes in Mac OS X. For example, in addition to running the Application Switcher, the Dock also runs all of the Dashboard widgets. That’s why the way to make all Dashboard widgets quit is to quit the Dock. (When you simply exit Dashboard, its widgets don’t actually quit—they keep running invisibly in the background, taking up RAM and CPU power.) You can see how processes run within each other in Activity Monitor if you click the Show pop-up menu at the top of the window and choose All Processes, Hierarchically.
Now, of course, you can also fix this by logging out and back in, or you can restart the entire computer. But I often work with a lot of programs and documents open, and that’s why I look for ways to fix problems without having to close all my programs and windows and then set up the entire workspace again.