So your Ext3 root partition is corrupted. No matter how you try to log into your system, your system asks you for root password. But you are a good, obedient ubuntu user. And you do not have a root password set. You can press CTRL+D, but that just reboots. Are you boned?

No. problem. press ctrl+alt+delete. You will get a messy screen about having trouble launching your X session, but you will get a login towards the bottom of the screen. So just use your normal user login and password. after that, I use
sudo su