How to reset/restart Cinnamon if the panels become invisible

Question

This question is a port of this one applied to a similar but distinct issue:

Problem:

The Cinnamon panel (my system: Linux Mint 19.1) became "invisible", that is, it still responded to mouse-clicks but became completely transparent.

Cause:

Having plugged in an external HDMI monitor and having switched to tty1 with CTRL+ALT+F1 and back to tty7 with CTRL+ALT+F7 (where Cinnamon runs) in order to have the external monitor recognized by the system.

Answer 1

This answer is a port of this one applied to a similar but distinct issue:

To restart the Cinnamon Panel

1. type ALT+F2 to open the command box
2. and then type r and enter.