I'm no pro here, but I'll give you some ideas to try and dig deeper while you wait for the actual pros...
Can you ping anything? For instance, try typing
and see if it finds it. If not, try pinging yahoo's IP like this:
if the first didn't work but the second did, you have a DNS issue.
You may also check the files /var/log/messages to see if it is giving you any error info. Oh, and /var/log/dmesg give all sorts of hardware related messages, so check that for entries regarding your eth0.
Let us know if this gives you any helpful info...