Okay, a few questions;
You had a module (aka: "driver") loaded for the NIC, right?
You can see that NIC when you do an "ifconfig -a", right?
See my post over in another thread
for more information about Realtek drivers.
As for your immediate problem (unbootable system); if possible first physically remove the RTL8168. Then grab the installation media and reinstall the OS.
Make sure you tell it to overwrite/reformat the filesystem(s), so that it will destroy the old Operating System (and all your data along with it).
That should do the trick!
P.S: There's got to be a joke in here about "Ubuntu" and "Unbootable", but I just can't find it...