Yeah, the ubuntu forums are probably faster. Stuff to check, however, would be:
`ifconfig -a` should show whatever interface you're using, configured, up, with an IP address.
`route` should show an appropriate default route entry pointing to your gateway.
From there, I usually try pinging the gateway by IP (or any other machine on the local segment), then pinging a remote host by IP, (184.108.40.206, BTW, is an L3 DNS server). If *that* works name resolution might be broken, which `nslookup google.com` will help address. Ensure you can actually ping the dns server; if you can't, tracepath will help identify the problem.
The other side of things is to just check and see what nm-applet (the NetworkManager notification area widget) does when you tell it to connect. Watching /var/log/messages may reveal no suitable interface. If this problem occurs on boot, you might check to make sure that lspci shows your ethernet hardware, modprobe -l shows the appropriate module loaded for your card, and so forth. Sometimes suspend just fucks things up, and a modprobe -r ; modprobe can kick it enough to come alive again.
As you may have gathered, all of this depends on your computer, network hardware, etc, so it's hard to just give you an answer.