On Mac:

which virtualbox 
sudo PATH/VirtualBox restart

Edit: Previous solution assumes that VirtualBox is started with the system while following is more generic:

which virtualbox 
sudo "PATH//VirtualBoxStartup.sh" restart
 
Done!!
Wednesday, January 23, 2019 - 11:58 Submitted 5 days 13 hours ago by Mutasem Elayyoub.