The security philosophy of Ubuntu excludes logging in as root. It is possible that your tweak to do so was lost with the upgrade to a new version.
Are you able to log in with your normal user account? In that case I suggest that you try using sudo to obtain root privileges. The advantage of sudo is that you get only temporary root privileges. This reduces the risk to destroy the system or delete important user data.
If you have problems to log in at all or to use sudo, I was going to suggest the following link.
http://www.psychocats.net/ubuntu/resetpassword
But I'm late and now I know that you already tried that.
Is your home directory encrypted?
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