when you install or changed the password to my MySQL database on web hosting control panel. you maybe meet "This site is temporarily unavailable.Please notify the System Administrator.Could not connect to the database server",it's a connection issue, which could be:
wrong username / password
mysql is off or not installed on the same server
if on an external server, firewalls could prevent access.
This might be a wrong mysql version, check the requirements.
on some virtual servers, the database includes a preset prefix like: username_dbname

to solve this make sure you put the correct database localhost and username and database name.

FTP to your server and open the configuration.php file. Scan through the code and you will see an item that says "mosConfig_password = XXXXXXXX ". That is the mySQL password your Joomla site uses. If you update that password with the one you changed on your joomla database , it should work again.