please need help my site is down

Hi there
I’m getting error and warning message when I try to access my site
It say’s: Warning: require(./wp-blog-header.php) [function.require]: failed to open stream: No such file or directory in \\boswinfs04\home\users\web\b2721\whl.lkayes\index.php on line 18
Fatal error: require() [function.require]: Failed opening required ‘./wp-blog-header.php’ (include_path=’.;c:\php\5\pear’) in \\boswinfs04\home\users\web\b2721\whl.lkayes\index.php on line 18
the person who created my site is not available and I need your help
I thank you in advance