While moving the Asset Store window, it suddenly jumped to a location where the top bar is off screen. That makes it uncontrollable, immovable, and useless.
The standard Windows keyboard shortcuts do not work for grabbing and moving the window. Unity Window Layouts do not affect this window.
How do I regain control?
I’m afraid you can never use the asset store again… 
Just kidding. Um… here: Where does Unity store its per user configuration info? - Questions & Answers - Unity Discussions
I believe that’s Mac path, but you should be able to locate something similar on Windows - probably one of the folders in %appdata% under Unity.
Right click your Taskbar (Start bar) and select Cascade, or side by side, or stacked. Or a mixture of the three. One of these should move the windows around enough for you to grab it again.
Something else that will work… Change the resolution of your desktop and back again. It usually jiggles everything around to different positions and resizes windows.