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?
1) locate the application that is hiding
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
2) move your mouse over the Thumbnail Preview window
3) right-click, choose Maximize
4) the window is now maximized, resize it and/or drag it to where you want it
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