Your server was recently upgrade to PHP 5.4 and it looks like the session_unregister() function no longer exists in that version.
I've gone ahead and set your account back to PHP 5.3 by modifying the .htaccess file in your home directory with this line:
AddHandler application/x-httpd-php53 .php
While reviewing your site, I also noticed that on your example.com/page1.php page, you have hundreds of spam comments there. I would strongly recommend you to log into your admin and remove these so they don't hurt your site's ranking with search engines.
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