I've been looking in Plasma's notification settings and since a few days, I am stuck.
The notification applet is there allright in the panel, but its icon does not show up. There is now an empty space in the panel, in my case between the KAlarm and the net_applet. It is there becausee when I click on it, the notifications pop up, or with right-click the setting options pop up. But the notifications only pop up when I click on the invisible applet icon. I'd liked to return to the default settings, but it eludes me.