This apparently was caused by an hplib update. This posting over on the Linux Mint forum resolves the problem.
https://forums.linuxmint.com/viewtopic.php?p=1622330#p1622330All the applets etc have now returned.
I do not know what I did except an update and I am always quite careful about that.
I appear to have lost all my Cinnamon 'System Settings' apps like 'display', 'desklets', 'add-applet-to-panel ......
Does anyone have any idea how I can recover these? re-install did not work.