If you accidentally remove your "generate sitemap" job, you can restore it by running this query in phpMyAdmin (or whatever tool you use to access your database):

INSERT INTO `Jobs` (`jName`, `jDescription`, `jDateInstalled`, `jDateLastRun`, `pkgID`, `jLastStatusText`, `jLastStatusCode`, `jStatus`, `jHandle`, `jNotUninstallable`)
VALUES ('Generate Sitemap File', 'Generate the sitemap.xml file that search engines use to crawl your site.', '', NULL, 0, NULL, 0, 'ENABLED', 'generate_sitemap' ,0 );