The Tracer dialog doesn't appear, but the main window becomes unresponsive.

This is most likely because the window is off-screen.

Solution:

1. Click the application's icon on the Taskbar to bring it into focus (even off-screen windows show up there).
2. Press Win+Arrow keys to snap the window back into view.

On multi-monitor setups, repeating the shortcut will cycle the window between displays.