I have an installation of 10.10 that has been working fine since I upgraded. One day I booted it and there was no network connectivity. The network indicator now says "Networking disabled". If I go to System > Administration > Network Tools > Devices the loopback is the default and though Eth0 shows up, if I select it everything is blank in the configuration and there is nothing in the interface statistics. lspci shows

Intel Corporation N10/ICH 7 Family LAN Controller (rev 01)

I have another card handy but would like to learn how to diagnose things like this. For example Is there a software reason that a working card would just stop working? Can the card be so visible but be totally broken? What steps can I try to "Enable" the card and networking or is there something that would point to hardware right off?

UPDATE: It is now working but I would accept the best explanation that might help someone else of why it is working or why I'm still in trouble.

Originally the only thing that showed up in the network indicator was the disabled text and there was no "Enable Networking". Based on another post I tried sudo dhclient eth0 though I have no idea what it does. It returned an ip address and I could browse to specific local ips. Then I rebooted to see what would happen and I had the same original problem so I did dhclient again but this time noticed that "Enable Networking" was now showing up in the indicator so I checked it. This time I rebooted and everything is working fine. But I still don't know why it stopped or why the steps I took worked.