Cannot connect to Database server (mysql workbench)

The issue is likely due to socket authentication being enabled for the root user by default when no password is set, during the upgrade to ubuntu 16.04.

The solution is to revert back to native password authentication. You can do this by logging in to MySQL using socket authentication by doing:

sudo mysql -u root

Once logged in:

ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'password';

which will revert back to the native (old default) password authentication.

Now use password as the password whenever required by MySQL.