It happened to me yesterday. I've been uninstalling packages from my linode trying to see what I can live without. Apparently Debian's /etc/init.d/networking can't bring up an interface without the ifupdown package installed.

Knowing's half the battle, brought about by a test reboot to see if things still worked.

Logging in via lish and reinstalling ifupdown is the other half.

Wow, what a derail huh.