Ubuntu Network suddenly becomes unreachable

This is a problem that I recently ran into on my computer running Ubuntu 10.04. I had shut down the computer and when I restarted it my network suddenly wouldn’t work. Pinging the router (in my case 192.168.200.1) revealed an error saying that the network was unreachable. After this I ran the ifconfig command to see exactly what was going on. Ideally I should have seen a section for my eth0 networking card, but the only one that was showing was the lo network, which is simply the local machine. The fix to this is actually quite simple. Open up the terminal and type in:

sudo vi /etc/network/interfaces

This is the file that controls all of your network interfaces. You should have a section for the eth0 card (most likely set up to use DHCP to control the network, but in this case there was only the lo section. All you need to do is add the following lines to the file:

auto eth0

iface eth0 inet dhcp

This will configure Ubuntu to use the eth0 interface running DHCP. Now run the following command to restart networking:

sudo /etc/init.d/networking restart

This should solve the problem, though I am not certain what the original cause was.