I looked this up because I had an issue where I couldn't open any KDE settings menus, but I didn't really want to reboot or log out. I found that the best way to do it (in KDE 5 at least) is by using krunner (Alt+F2) to killall plasmashell then immediately afterwards, plasmashell. I tried doing it in a Konsole window, using killall plasmashell and then plasmashell & and this worked, but as soon as I closed the Konsole window, plasma would close as well. Even though you seemed to resolve your issue, I figured I would post it for people coming along in the future needing to restart plasma.