You open your Windows application/program and it is not visible because it has opened outside of your monitors area. Or it is slightly visible but you can't move it, as you can't grab the title bar. This can happen if you have a laptop that was using an external monitor, where the application was last opened on, and currently that extra monitor is missing from your set-up.
Open your application and make sure you have focus on it, by click on it in the system. Then hold down Alt + Space-bar and then press the M key, then let go of all keys. Now you can use the arrow keys to move the window back on screen. If you can't see the Window at all, try and remember where is was when your extra monitor was attached, so you know roughly which way to move the window.