A Site Temporarily Unavailable Error 2 indicates that Joomla is having some problem with the database. If it happens with a change (updating) of mysql, it could be that the password is in trouble. In my case, I could still use mysql-front end to connect to the database, but not with phpBB and Joomla. Here is how you can solve this type of problem...
At lease phpBB indicated that "Client does not support authentication protocol requested by server, consider upgrading MySQL client" while Joomla didn't say what's wrong and kept me guessing.
The following is taken from MySQL 5.0 Reference Manual.
MySQL 5.0 uses an authentication protocol based on a password hashing algorithm that is incompatible with that used by older (pre-4.1) clients. If you upgrade the server from 4.0, attempts to connect to it with an older client may fail with the following message:
shell> mysql
Client does not support authentication protocol requested by server; consider upgrading MySQL client
To solve this problem, you should use one of the following approaches:
Upgrade all client programs to use a 4.1.1 or newer client library.
When connecting to the server with a pre-4.1 client program, use an account that still has a pre-4.1-style password.
Reset the password to pre-4.1 style for each user that needs to use a pre-4.1 client program. This can be done using the SET PASSWORD statement and the OLD_PASSWORD() function:
mysql> SET PASSWORD FOR
-> 'some_user'@'some_host' = OLD_PASSWORD('newpwd');
Alternatively, use UPDATE and FLUSH PRIVILEGES:
mysql> UPDATE mysql.user SET Password = OLD_PASSWORD('newpwd')-> WHERE Host = '<em>some_host</em>' AND User = '<em>some_user</em>';</strong><br /> mysql> <strong>FLUSH PRIVILEGES;</strong></span></p> <p><span style="font-size: 10pt; font-family: arial,helvetica,sans-serif; color: #000000;"> Substitute the password you want to use for “<em>newpwd</em>” in the preceding examples. MySQL cannot tell you what the original password was, so you'll need to pick a new one. </span></p> </li> </ul> <p><span style="font-size: 10pt; font-family: arial,helvetica,sans-serif; color: #000000;"> </span></p> <hr /> <p><strong>NB</strong>. The number following the message indicates a database problem. The error number is generated by Joomla and the error numbers 1, 2 and 3 indicate the following:</p> <ul> <li>Error 1 = FATAL ERROR: MySQL support not available: Joomla attempts to talk with mysql on the server and fails to even talk to it. This might be because the MySQL server is down. </li> </ul> <ul> <li> Error 2 = FATAL ERROR: Connection to database server failed. Although MySQL is installed but for some reason Joomla cannot login (this is where it tries the host, username, password stuff). </li> </ul> <ul> <li> Error 3 = FATAL ERROR: Database not found. Joomla can connect to MySQL but cannot select the specific database.</li> </ul> <p> </p>'