I cannot log into the Joomla! admin.
Updating the password in PHP My Admin does not work.
It always says that my username and password do not match, the user does not exist or I have not been activated yet.
In this instance I had multiple users tables in my database. This was due to either a backup and restore, or an upgrade from 1.5 or 2.5.
I manually deleted all of the tables that started with the wrong prefix.
This left only the currently prefixed tables (I checked in my configuration.php to see which one was correct) and was then able to use the users table to update my password successfully.