I once spent 3 days troubleshooting a windows 2000 machine that wouldn't connect to the LAN. I reinstalled windows half a dozen times, verified the router was working a dozen more, checked all the IRQ's and DMA channels (shows how long ago this was) and everything else I could think of. No matter what I did it wouldn't connect. And yes... I DID check to see if the LAN cable was plugged in.
I finally decided to swap out the LAN cable even though it had been working earlier.
Turns out I had the end of one cable plugged into the NIC card - the end of a DIFFERENT cable plugged into the wall.