I just tried Gnome 3, but didn't like it at all. So I removed/purged it, rebooted, and discovered that my Nautilus looks different now & lacks a menu. I couldn't even find a way to bring it back to it's normal appearance. How do I get it back to normal without reinstalling the whole OS? I'm very unhappy with this, since there is missing functionality (or at least it's hidden and I can't find it)!
ubuntu-desktop depends on
nautilus, you can and should re-install that as well to avoid future problems:
sudo apt-get install -f ubuntu-desktop
This will install Nautilus as a side-effect.
You can also just reinstall Nautilus with the following command:
sudo apt-get install --reinstall nautilus