I had the same problem with openSUSE 42.3 as I do now with openSUSE Leap 15: occasionally the plasma workspace would freeze and the only solution would be to hit "Alt + F2" and open Konsole and use the following command:

kquitapp5 plasmashell && kstart plasmashell
It occurs utterly random so I realise this makes it difficult to diagnose, but maybe someone has an idea?