You could login as root, but Ubuntu locks the root account by default and you have probably uninstalled the tools needed to enable it. You could possibly find a su binary and just add it to /usr/bin/su. But you would need to add all the dependencies from scratch.

You could follow the advice here and use a chroot environment with a liveCD to reinstall libc6.

I think your best bet, unless you want to spend hours scraping together tar files, is to reinstall your system and hope that you have current backups.

There is a post on serverfault about this problem. But unless you have a lot of time on your hands I would just reinstall.