When using multiple monitors with a laptop or workstation or even changing monitors, sometimes an application will have been closed in a position that does not exist anymore. You may or may not be able to see part of the window, but you are unable to grab the top of the window to move it.
There are two ways I have found to get it back. First, you can try right clicking on the taskbar icon. If 'move' is an option, click move and use your arrow keys to move the window.
If 'move' is not an option, in trillian for example, right click on your desktop and change your monitor resolution to something smaller while the application is open. It should 'pop' back into the available desktop. You can then put your monitor resolution back how you prefer it and you should retain control of the application window.