I used external extra monitor fo PI because it was bigger, later I disconnected it and started Particle Illusion, but PI still wanted to host the program on the (now detached) extra monitor.
Try this:(On Mac)
Use extra monitor
Drag PI over there.
Detach extra monitor
> Can you actually get it to appear on the main screen? I couldn't.
Could be really bad news if it was crunch time and it was located offscreen. I guess I'd try deleting the prefs file for a start.