Check if wp-config.php exists
Have a quick 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
Open wp-config.php file in editor and make sure that database name, username, password are correct. Check if table prefix in wp-config.php file matches with what you have in your database (case-sensitive).
Check wp_options table
Navigate to wp_options table and check values of the siteurl and home. You may need to update them if your domain name has changed.
Try database table repair
Try repairing WordPress database tables by referring to our knowledgbease article.
Restore WordPress database
If any of above solutions do not work for you, try restoring WordPress database from the latest backup you have. You can also ask your web host to restore the WordPress database for you. Please share if you have any other solutons that worked for you.