In most cases, the MySQL login credentials are incorrect (MySQL login, password or host or sometimes the database name as well). Double check the phplive/web/config.php file and check the SQL variables. If the variables contain the correct credentials, then the actual credentials were changed at the MySQL database. Contact your hosting company or server admin to obtain the new MySQL login credentials or to see if anything has changed.
Documentation on updating the config.php file to the new MySQL values.