It may be that eth0 has not been placed in the "up" state by the system (for some reason). What is the output if you run the following?
Code:
sudo ifconfig eth0 192.168.1.100 netmask 255.255.255.0
ifconfig
If eth0 is now shown, then it means it is not being configured at boot. This could be done either through a desktop network config applet (e.g. System > Administration > Network in Gnome) or via the interfaces config file (/etc/network interfaces in Debian derivatives).
If not, it probably means the driver is either not loaded, or missing. please post the output of
lsmod, and the section of
lspci -v relating to your network card. It would also be helpful to know which distro you're using.
Hope this helps,
Rob
