First step is to identify whether you are using the correct driver for your net card.
as root will list the modules.
do some research as to what the correct module you should be using is, and write down or print out what you find out.
Alright, now depending on your bootloader, you should be able to get into single-user mode without problems.
Do this by using the command
at the lilo boot prompt.
That should boot you into single user mode (root access, be careful it can be dangerous
Now poke around in the /etc/rc.d/ directory and look through the scripts to see which one is loading your network interface. It'll probably be rc.M or rc.4 or rc.inet1 rc.inet2 etc.
M means multiuser mode, 4, is the same thing -- it varies by distribution.
You can use the command
to read what's in the file and
to edit it.
Add comments like this to comment out the section that sets up the net interface on boot --
change that to...
Adding the "#" sign comments out lines, and they are ignored.
Now, reboot and you should be able to get into multiuser mode, but without ethernet at this point.
Use the info you learned about your card and the correct driver, try to load the module with
modprobe <module name>
the other option would be to boot from another distrobution -- based on floppy disc or cd, and muck around from there to see where the problem is if you can't boot into single user mode.