Do you have a network icon anywhere on your panel? If networking is down it might have a red "x" over it. If not, you should be able to search you menu for "network connections" or "network manager".
If you do have an icon on the panel, you can left click on it and look for a check box with "Enable Networking", make sure it's checked. If it wasn't, fixing your problem may be a simple as that.
If you don't see an icon, look in the menu for Network Connections, click on wired, then click on the eth0 and then edit, and make sure the box is checked next to "Connect Automatically" and "System Connection".
If it's a wireless connection, check the boxes in Network Connections under Wireless".
I'm hoping someone with an Ubuntu install can check what I've just written. I use KDE on Fedora, and, while Network Manager is similar on both Gnome and KDE, there might be differences that they can point out.