I am receiving phpBB: Critical Error Could not connect to the database error.This error means that you have not configured your PhpBB's database settings properly. Revise the configuration file of phpBB which is config.php located in the directory where your forum resides.
Please note that your database names and usernames always begin the following prefix "user_" where user stands for your username. For more information about Mysql management check this video tutorial.
Another reason for this error is when your database has been deleted. In this case you will have to restore it from a backup. If you don't have a local backup you can use Siteground's.