Perhaps if you start a terminal window and type "lspci", you'll get
some information that will help you. On a machine I recently had the
same problem with, I get a line like this (after some other stuff):

01:0e.0 Ethernet controller: ADMtek NC100 Network Everywhere Fast
Ethernet 10/100 (rev 11)

It's an old HP that I recently moved from one location to another and
upgraded from 6.06 to 8.04 myself. It did not recognize that there was
an Ethernet card, and that line was not in the output of lspci after I
moved the machine and upgraded. I got it back by taking the cover off
the machine and reseating (removing and reinserting) the Ethernet
card.

It's a real long shot that you'll have the same cause to your problem,
but having seen the same thing recently, I thought I'd see if the
information helped you.