Well, in that case, your Plasma session probably crashed, which, although not everyone will experience that, is no surprise, really.
This Stable Update contained an update to Plasma itself, from 6.3.4 to 6.3.5 — see the first post of the thread. So the shared libraries that were going to be updated were in use while they were being overwritten.
I know nobody wants to hear it, but the safest way to apply a bundled update — especially if it involves your desktop environment — is to run it from the command line in a tty while completely logged out of the GUI environment. See… 