Inspecting the site with FireFox and FireBug, your index.php seems to be missing the reference to the logo.
<a href="<?php echo $this->baseurl; ?>" class="nounder"><img src="<?php echo $this->baseurl; ?>/templates/<?php echo $this->template?>/images/blank.gif" style="border:0;" alt="" id="logo" /></a>
<?php if ($this->countModules('top')) : ?>
To resolve this restore fro ma recent backup or replace the index.php with a copy from the template package.
I can't say with certainty, but unless you've edited your index.php, you may be been hacked.
What version of Joomla are you running?
Be sure you're running at least 1.5.6, 1.5.8 is the latest.