You may already seen this annoying message instead of your Joomla site:
This site is temporarily unavailable.
Please notify the System Administrator.
And you are virtually clueless what to do about, and even the smallest bit of info on what happened can be a lifesaver.
The above error message may appear after you have installed Joomla for the first time, or when you have uploaded or moved a copy of your website from one server or domain to another.
This error occurs generally when Joomla cannot log in to the MySQL database. It could be that there is a problem with the database, or more likely, the connection details supplied in your configuration.php file are incorrect. Underneath the above message, you will find a number: either 1, 2, or 3. This number can help you determine why Joomla cannot access the database:
Error 1: MySQL support not available (Either MySQL is not installed, or PHP does not have the correct modules loaded).
Error 2: Connection to database server failed (The most common one - usually means that the host, username, or password values in configuration.php are wrong).
Error 3 = FATAL ERROR: Database not found (The connection details are ok, but the database name specified in configuration.php does not exist).
Check the host name, database name, user name, and password in your configuration.php file. Check that a MySQL database exists on the server with the given name, and that the user name and password supplied in configuration.php has permission to read and write on that database.