This means that the website can not access the database that is assigned too. To fix it try:
- Check error logs first: /usr/local/cpanel/logs/error_log
- Incorrect login credentials. Database password has been changed but the password in "/public_html/configuration.php" or "/public_html/wp-config.php" has not been changed. This depends on what website platform you are using.
- Corrupted database
Try uploading the old database from the backup, if the problem still occurs contact support.