Sounds like a driver issue, are there any proprietary drivers that need installed? (check the top bar, an icon next to the mail might be there...) A couple other troubleshooting ideas; Make sure lights are on by the ethernet plug on both the router/switch and the back of the computer. Try another ethernet cable. Try moving the NIC to a different PCI slot if you can. And last but not least, try a different card or plug in a wireless dongle if you have one.
I hope one of these solutions solve it
otherwise try downgrading to the next lower version of Ubuntu and see if that works.
