Please provide more information, such as the make/model of the computer, whether you are connecting wired or wireless, and the make/model of your ethernet adapter (you can use the
lspci command from a terminal to determine that).
Also, if using a cable, test the cable and try a different port on your router or hub. Cables and ports do go bad, and, if it's the cable, all the software in the world won't fix it.
Here are some other troubleshooting tips:
http://www.ubuntugeek.com/ubuntu-net...ting-tips.html
If they don't help, then, in addition to the above, post the output of
ifconfig -a and of
/cat /etc/network/interfaces, being sure to surround it with "code" tags, which become available when you click the "Go Advanced" button to the right of the "Post Quick Reply" button at the bottom of the reply window.