I recently had the same problem. Even re-installing the program didn't change anything. It only happened to one scenario for me. And I don't know what caused it. If you have the same problem, then *somehow* your window is outside of your screen. If you open a different scenario (if you have any), then it should be on your screen. So what I did was that I copied all files to this new scenario. However, there is an easier way: To fix it, open the project.greenfoot-file which opens your scenario in a text editor while the scenario is closed. Change the last lines to: This is the position of the top left corner of the main window.