It's way too easy to accidentally drag and drop a windowed-application to the dead-zone, behind the Taskbar across the bottom. With a mouse, or with a touchscreen. See the video below for a demonstration of the problem and solution, using your mouse cursor.
It's easy enough to just kill the window, which even Task Manager can handle. Or just click the tiny red x from the Thumbnail preview window:
But what if you actually want to see what's going on in that window, or save your work?
Simply move your cursor along the Taskbar icons, starting with the right-most icon, to locate the running app that has it's menu hidden below the Taskbar
That's it, that simple!
Here's a more advanced, thorough video demonstration:
If you like the giant size Thumbnails seen in this article and video, see also:
Change Windows 7/8 Taskbar Thumbnail Preview Size