You are running Windows 7 on a notebook or Tablet PC computer.
You change the display orientation of Windows using the keyboard shortcut: Ctrl+Alt+<Arrow Key>.
In this scenario, the Windows taskbar may stay in its previous location and not properly rotate with the newly configured orientation. In addition, subsequent changes to the Windows orientation will result in the same behavior.
If the mouse button is accidentally being held down with the cursor on the taskbar at the same time the Windows orientation is changed, you may see this behavior.
When changing the Windows display orientation, ensure that the mouse button is not being held down at the same time.
To work around this issue, you can use the mouse to drag the taskbar to the desired location on the desktop. Note that in order to drag the taskbar, you may have to first unlock it from its current location by right-clicking on the taskbar and in the context menu, removing the check next to the "Lock the taskbar" option.