Thank you! I was confused by the fact I was not using dual monitors at the moment the problem occured (it was actually already a week ago that I used dual monitors), but your suggested solution fixed the problem! I followed the steps: 1. Press Alt+Spacebar. 2. Press the down arrow. 3. Press Enter (this action will put the window into move mode). 4. Press the right arrow key (this action will tell the OS that you are moving the window). 5. Move your mouse around and see whether the window becomes visible. And the window of the query builder became visible! Thank you! ... View more