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?
sudo ifconfig eth0 192.168.1.100 netmask 255.255.255.0
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,