Depending on the level of “got stuck”, one option is to press Ctrl+Alt+Esc (more on the shortcut below) and it should bring up a skull and crossbones cursor.
From there then you can either click on an application, assuming it’s not so stuck that you can’t move your mouse, or Enter to make the application under the cursor Force Quit. You can dismiss this by pressing Escape.
Important note: If you have the desktop under the cursor, it will kill plasmashell (I just discovered while testing) and you’d have to access KRunner (normally either Alt+F2 or Alt+Space) and run a command depending on which version of KDE:
- KDE Plasma 5:
kstart5 plasmashell - KDE Plasma 6:
kstart plasmashell
Essentially be careful where you click.
Regarding the shortcut, you can actually double-check the key binding and set it to something else yourself if you go to the System Settings and under Shortcuts, then under KWin, there should be a “Kill Window” shortcut.