An up and running operating system exposes a number of services to the user or the network by running so called deamons in the background. These are the services you may be familiar with from the Microsoft Windows envirnment, where the Task Manager (opened with <CTRL>+<ALT>+<DEL>) lists all sorts of little programms humming away.
No, under some circumstances, one of these services/daemons might get into problems, freeze or need a kick in the butt to reload configurations or update. This can be done by re-running the exact same script again the system executed on boot time to start the service/daemon in the first place.
To restart the networking sub-functionalities try this
sudo /etc/init.d/networking restart
This command is for an Ubuntu/Debian system. Your mileage may vary on other systems. To get available options try this this
which doesn't pass a command to the scripts and thus returns available options. For example, if you got apache2 running, the command
sudo /etc/init.d/apache2 force-reload
will restart apache2 and reload the configurations file so changes take effect.