If your KDE Plasma environment is hanging or freezing you don’t necessarily have to logout and login. Open the Search/Run prompt by pressing ALT+F2 and enter konsole to open your terminal.
Then simply issue the necessary command to restart plasma shell.
For KDE4:
killall plasma-desktop
kstart plasma-desktop
KDE5:
killall plasmashell
kstart plasmashell
KDE5.10 and above:
kquitapp5 plasmashell
kstart5 plasmashell
Credits goes out to Korcia at this AskUbuntu thread.