@alit: thanks for the advice
@leonard Smallbones: you should first check the wp-config.php file to see if DB_NAME, DB_USER, DB_PASSWORD and DB_HOST are set to the correct values. Also maybe the db was not imported correctly, if you can you could try to do a reimport.
WordPress also has a way to repair damaged db tables….. just insert define('WP_ALLOW_REPAIR', true);
in wp-config.php and then go to https://www.yoursite.com/wp-admin/maint/repair.php and follow the steps to repair database.