problems with get_header, get_footer, etc.

Hi there! I am a bit new to WordPress, so please forgive me for my inexperience and lack of knowledge.

Basically, my index.php runs fine, I can call the header.php, footer.php and sidebar.php fine — however, in other pages (for example, I have a contact details page), I am experiencing fatal errors.

“Fatal error: Call to undefined function get_header()”

I’m probably missing something really simple and obvious, but I just can’t figure it out.

This is the code I am using to call the header in –

<?php get_header(); ?>

Any help would be super appreciated!