I'm running a new install of Kubuntu 10.04. I installed it a week ago. I'm using wired ethernet. Yesterday everything was working fine. I have not made any changes other than installing some updates yesterday. Today I have no network interfaces listed in ifconfig except lo.

But my network cards are listed in lspci.

What are the next troubleshooting steps? Thanks

EDIT:
ifup ethX reports the interface is unknown.

cat /etc/network/interfaces
shows only the lo interface, not eth0 or eth1.

If using the GUI to launch KNetworkManager, the application doesn't open. Nothing happens. No error, but KNetworkManager just doesn't open.
(I've only been using KDE a short while. I'm not used to any of the tools.)

grep -i eth /var/log/messages
doesn't show any obvious errors. I can't paste the file at the moment because that machine has no network and the memory card reader isn't working (a hardware problem that's existed for a long time).

Should show you all of your ethernet interfaces. If it's not there then it isn't being detected.

If not does dmesg show you any problems. Scroll down and look for your device
If it shows up in dmesg then maybe you changed your kernel module with that update. Maybe you can roll back. If it's not listed in dmesg then it isn't working.

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eth0 and eth1 are listed when I use ifconfig -a. I normally only use eth1. At the moment, neither has an IP address.

dmesg doesn't show any obvious problems.

I think what happened is that when I stopped work yesterday I hibernated my system (which I don't normally do and which I don't think my hardware handles well). It failed to resume from that state, but it did boot up fine.

Is it possible that the interfaces were "taken down" or disabled by the hibernation and then not restored properly? If so, how would I tell that? And how would I resolve it?

Some devices don't wake up correctly. Look at acpi for your distribution. I've never used Ubuntu.

On this FreeBSD machine the mouse will drop out after sleep and wake up. I solved it by adding
to /etc/rc.resume

So yes that's quite possible that the network card is not comming back after sleep.

Thanks for your help. I got it working. I edited the /etc/network/interfaces file and added these lines:
auto eth1
iface eth1 inet dhcp

then I did /etc/init.d/networking restart
and my network was back.

It seems really strange to me that hibernating my system would cause the removal of lines from the interfaces file. Could that be what happened? If so, that's just too weird.

Now I am wondering what else might have gotten screwed up.Maybe I need to check /etc/resolv.conf and some other files... any suggestions on how to ensure my system is running properly?

Should I post a log file here? I don't see any obvious errors (but that could just be due to the fact that I don't know what to look for). Thanks.

EDIT:
This is a new installation. If it is screwed up, I would prefer to either fix it or reinstall from scratch before I invest all the time to customize it to my liking. That's why I'd like to know if it has any other problems now, before I move forward. What is the best way to find any and all error messages from the various log files?