I use a laptop (Windows 10) and occasionally use a second external monitor to extend my screen. You can replicate the problem I found with the following steps.
- Put Tropy window full screen in the external monitor.
- Quit Tropy.
- Disconnect external monitor.
- Restart Tropy.
At this point, Tropy will be running (and will show up in the taskbar), but it will not be visible because it still thinks it is located in the now-non-existent extended monitor screen space.
My extended monitor is placed to the right of my primary screen. So if I press [Windows Key] + [Left Arrow Key] (the keyboard shortcut for moving windows from one screen to the other) I can make it appear once again in on my main laptop screen (if your external monitor was to the left of your primary screen you would use the Right Arrow Key).