Hi
It sounds like you wound up with index.php from a theme in the location that the main WP index.php is supposed to go.
The code for the main WP index.php that goes in the root folder is
<?php
/**
* Front to the WordPress application. This file doesn't do anything, but loads
* wp-blog-header.php which does and tells WordPress to load the theme.
*
* @package WordPress
*/
/**
* Tells WordPress to load the WordPress theme and output it.
*
* @var bool
*/
define('WP_USE_THEMES', true);
/** Loads the WordPress Environment and Template */
require('./wp-blog-header.php');
?>
As you can see, there is no call to get_header() on line 1. get_header() is a function used in a theme, not in the core WP file.
Confirm whether what I am saying is so, and if it is, figure out how your theme file got into the root folder. Themes belong in /wp-content/themes/{themename}/