Database connection error
There are multiple names for a "Database connection error", here are some of the main ways to trouble shoot that error.
For most cms(s) these steps will work. If none of this help I would google the error with the name of the cms.
For more information on the default configuration file for the popular cms click here
1. Verify the database name, username, password, and host.
2. Can you reach /wp-admin on your site? Your database may need to be repaired.
3. Check to make sure MySql is responsive
4. Check the options table in PHPMyAdmin and the rows called "site_url" and "home_url". Make sure they go to the correct domain name.
5. Verify that your user you are using has access and permission to that specific database.