Bitrix auth script (if you don't know admin password). Create this file in the site root directory.

bitrix-auth.php

<?php
require($_SERVER['DOCUMENT_ROOT'] . '/bitrix/header.php');

global $USER;
// auth user with id = 1
$USER->Authorize(1);

// delete this script from server
@unlink(__FILE__);

LocalRedirect('/bitrix/admin/');

require($_SERVER['DOCUMENT_ROOT'] . '/bitrix/footer.php');