How to recover dialogs that don't show on screen
Several options, pick one of the following:
- Reconnect that 2nd screen you might have had connected at one time and moved that dialog to.
- Revert to factory settings (easiest)
- Install and use the plugin https://musescore.org/en/project/reset-dialog-positions (less invasive, available for MuseScore 2 and 3)
Use some Microsoft Windows magic to move the dialog (Windows and language dependant):
- Put your mouse pointer near the middle of the screen
- Press Alt+Space
- Press M (English, for "Move", German: V, for "Verschieben")
- Press any arrow key
- Click mouse
Some more Windows magic: Windows Key+→ (or ←). Attaches the dialog to the right (or left) border of the screen.
- Change the scaling in your display settings to something smaller (like from 150% to 125%)
- Press Esc to leave that invisible dialog (at least solves an apparent 'frozen' MuseScore)