It would be most helpful to tell us what distribution of linux you are using.
Start out by trying '/sbin/ifconfig'. If it shows an IP assigned to your 'eth0' interface, then you probably have a cabling issue. Otherwise, if your distro is something Redhat-ish, try, as root, '/sbin/ifup eth0'. If that still doesn't work, get some details about your ethernet hardware, and report back here with the output from 'lsmod'.