Uncaught ReferenceError: $ is not defined?

How come this code throws an

Uncaught ReferenceError: $ is not defined

when it was OK before?

$(document).ready(function() {
    $('#tabs > ul').tabs({ fx: { opacity: 'toggle' } });
    $('#featuredvid > ul').tabs();
});

Results in tabs don’t close anymore.

jQuery is referenced in the header:

<script language="JavaScript" type="text/javascript" src="<?php echo get_option('siteurl') ?>/js/sprinkle.js"></script>
<script language="JavaScript" type="text/javascript" src="<?php echo get_option('siteurl') ?>/js/jquery-1.2.6.min.js"></script>
<script language="JavaScript" type="text/javascript" src="<?php echo get_option('siteurl') ?>