You are using a Windows 7 computer with a monitor attached via a DisplayPort connector. You turn the monitor off using the power button, then turn it back on.
In some cases, you may observe that some open windows and/or desktop icons have changed position. For example, if a window was at the lower right of the screen, it may have moved toward the upper left of the screen.
Some DisplayPort monitors, when powered off, may de-assert their Hotplug Detect signal, indicating to the computer that the monitor may have been unplugged. In some cases, when Windows 7 detects that all monitors have been unplugged, it will switch to a default display resolution of 1024x768. When this happens, any open windows and desktop icons may be moved or resized to fit within a virtual 1024x768 desktop.
One potential workaround for this problem is to avoid turning off the monitor manually via its power button. To conserve power, you can instead use the Power Options in the Windows control panel to set the monitor to be turned off automatically when the computer is not in use. To adjust this setting, click on the Start button, and in the "Search programs and files" text box, type "Choose when to turn off the display" and press Enter.
Alternately, the problem may not occur if you attach the monitor to the computer using a different type of display cable, such as an HDMI or DVI cable. Consult the manuals for your computer and your monitor to determine what types of display connectors are supported.