Driver problem ?
I have just experienced a similar problem with an Nforce 2 board and the nvnet driver (1.0-0261) : in the middle of a remote "apt-get update" (this machine is a .deb proxy), the network is suddenly down and no configuration (ifdown, ifup, ifconfig, rmmod, modprobe, etc.) could get it back working. Even a soft reboot did not solve my problem. I then discovered that the network card seemed to be in a strange state : the green light was constantly on (a good thing), but the orange one was constantly flashing at a very rapid pace. The only solution I have found so far is to shutdown and remove the power cord (there is not back power switch on my Shuttle SN45G) to reinitialize the card on reboot. Only after this, the orange led is constantly on (and not flashing) and the network is correctly configured.