Windows: Bringing lost or zero size windows into view

First, make sure that the program in question is active, either by using Alt-Tab to select it or by clicking the program's button on the taskbar. Now press Alt-Spacebar to open the system menu of the active window, the one you normally access by clicking the icon at the window's top left. The menu will appear along the edge nearest to the window's off-screen location. Press M to select the Move menu option and then use the arrow keys to bring the window back into view. Actually, once you've pressed any arrow key, the mouse should move the errant window even though you're not holding down a button.
If the window were truly zero-size, you could bring up the system menu in the same way and press S for Size. Press an Arrow key to select the corresponding edge of the window. Continue pressing the same Arrow key to expand the window in the direction of that edge.