If you use a multi-monitor setup, there are chances that you might forget to move the applications windows back to main monitor before disconnecting it. In such cases, when you try to access the window you might now be able to do it as it may still assume that it is on the secondary monitor.
Windows 7 does not have a direct option to move the window back to the main monitor using the taskbar. When you right-click on the taskbar icon you will only see the jumplist and not the regular options.
However, if you want to see the other options like Move, Maximize and Minimize, just hold the Shift button while you right click, this will in turn show you the other regular right click options. Once you do that you can easily move the window from the non-existent secondary monitor back to the main monitor using either the arrow keys or your mouse.