Can't connect to a wordpress database, so start by checking your wp-config.php file and making sure your database connection details are correct (and that there is a config file there!). Since it was probably working before, and isn't now, it might be because of a server change. Contact your hosting solution and check to see if they switched you out IP's, or possibly changed your MySQL connection details. If that's the case update your wp-config file.

Have you installed any new plugins recently, or updated old ones? Sometimes buggy plugins can cause these errors. If you think this could be the case, log into your web host's FTP service and try changing the name of the folder of the suspected plugin. WordPress may begin responding again.

Check your file "wp-config.php".
If you activate the plugin, remove your plugin from control panel, refresh and repair database table in phpmyadmin.

Check if the (MySQL) Database is still there. If so, reset password and enter the new one in the wp-config.php file