This can happen if you open a dialog and IDEA appears to freeze up. This will usually happen if you usually use two monitors and then detach one of them and only use one monitor. The problem is not that IDEA is locking up but the problem is that IDEA has opened the dialog in the monitor that is no longer attached to the computer so you can no longer see the dialog. How to get the dialog back, you must first hit Alt + Space at the same time, then hit M (l if you are using the French version of IDEA) and then you can use the cursor keys to move your dialog back onto the screen attached to your computer
Another option is to attach a second screen to your computer and then move it over a screen, but this is not always an option.