Volume control will often stop working, if your sound server (normally pulseaudio) dies or restarts whether by itself or by you. The reason pulseaudio may need to be restarted is due to some sort of crash or other issue that prevents sound from working (normally restarting or doing a killall pulseaudio fixes things).
However, you will normally find at least in OS's like Ubuntu/Mint 16/18+ that you cannot control the volume whether adjusting the level, changing input/outputs and muting.
This is because the instance of your volume control in the task manager was tied to the instance of pulseaudio at the time. Since pulseaudio was killed/restarted for some reason, the volume control applet doesn't work since it is trying to adjust the volume on an instance of pulseaudio that no longer exists.
To fix the issue of volume/sound settings not working, just kill and restart the applet:
If you are using another distro/flavor doing a "ps aux|grep volume-control" should get you the name of your volume control applet.
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