This implies WordPress is experiencing technicalities in accessing your database. One major cause is incorrect entry of essential details in the wp-config.php file. It can also occur when you surpass the data limits. This is mostly common with web share hosting plans. There is also a possibility of this error occurring when your server is technically down. Database connection errors also occur when websites have been hacked.
To correct the error, you will have to locate the wp-config.php file using PHP. Thereafter, you can make confirmations of the database credentials and ensure they are correct.
In case your web server is down, you might need to get in touch with your web host for rectifications. This is also applicable in cases where you have surpassed the allocated data quotas.