Fatal error when blog loads

Everything have been working fine up until now. All of the sudden I get this message when I try to load the blog:
Fatal error: Unknown function: get_thetitle() in /usr/home/whitehat_dk/users/android8/wordpress/wp-includes/template-functions-comment.php on line 292
Did a quick search in the forum, but didn’t find anything…
Any suggestions?
Thx. in advance!