Disappearing blog

How is it possible that a blog that was used only aweek ao cansimly just disappear from WordPress. When I try to access my blg I get the following error message: Fatal error: Cannot redeclare class Walker_Page in /data/16/2/138/121/2138447/user/2347011/htdocs/WordPress/wp-includes/post-template.php on line 1062

Can anyone help!!!!