This means that the page template category.php is fine, but not your main template, right now, there is no footer at all, did you remove or edit footer.php? or remove the call to the footer from a page template?
<?php get_footer(); ?>If you do not belief you edited any theme files, re-upload theme (or reinstall from dashboard) NOTE: keep a fresh copy of twentyeleven file sytle.css (keep your stylesheet, do not over write...) or you will lose any edits.