Frequently, Lr shifts on my screen and re-positions the window so that I can see neither the file menu pull down on the left or the expand/minimize/close boxes in the upper right. The only way I can regain the controls is to close the application, by right clicking on the Windows bar icon, and then reopening.
This only takes a second and it comes back to where it was, but why does it do this and how can you re-position it on the screen?