If your Dock stops responding (e.g. cannot click application icon) and you can’t switch between applications using ⌘ + ⇥ (Command + Tab) there is an easy way to deal with it without restart.

Just open Activity Monitor (typically ⌘ + ␣), find Dock process and kill process using “Quit Process” option. Dock should be restared after a second and everything should back to normal.