Windows has placed the screen outside of the monitor area. Press alt+spacebar (does the same thing as clicking on the upper left-hand corner of the window), then press 'M' (for Move) and you may be able to move it back onto the monitor using the arrow keys.

Hold down the Alt, then tap the spacebar.
Release the Alt key, tap the letter M.
You may see the 'Spacebar' menu at the edge of the screen somewhere.
Press the arrow keys to position the window. (Just like space invaders or Centipede.)