Sometimes in WordPress, for reasons unknown, jQuery does not get registered or loaded. This function checks if jQuery has been enqueued, and if not, registers and enqueues it.

load_jquery.php

function bfp_load_jquery_if_not_loading(){

	if( ! wp_script_is( 'jquery', 'enqueued') && ! is_admin() ){
		wp_deregister_script('jquery');
		wp_register_script('jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js', false, '1.11.3');
		wp_enqueue_script('jquery');
	}
}

add_action( 'wp_enqueue_scripts', 'bfp_load_jquery_if_not_loading');