Check file wp-config.php is exists
Please look into your wordpress files and make sure that wp-config.php does exist, also check this file is not misspelled in the name or the extension.
Check database connection details
Please open file wp-config.php and make sure that database name, username, password are correct. Also, check if table prefix in wp-config.php file matches what you have in your database.
Check wp_options table
Through PHPMyadmin go to wp_options table and check the values of the
WP_SITEURL . You have to update them if your domain name has changed.