After some poking around (which I don’t like) I ended up using dconf-editor.
It was no immediate help and it needs to be used very carefully to avoid contradicting settings etc.
So up to another search on the Net.
In a nutshell, the panel and its “entourage” (menu etc.) retained all my settings but the panel itself was missing.
<ctrl><alt>T brought up the terminal window (also with all its settings - yay!) and
/usr/share/cinnamon/cinnamon-settings/cinnamon-settings.py panel
launched the panel editor from the shell.
There “Add new panel” (bottom) was enough to get the missing panel and all my settings back! 
Source: https://forums.linuxmint.com/viewtopic.php?t=257031
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