site died after WP core update to 3.9

My site, RiBservices.com disappeared after the WP update. The error message seems to me to not be so serious (from the mouths of babes and fools, eh!) but the hosting service who originally installed WP refuses to do anything about it. The site was originally running on ProConsultant Theme from PremoPress … any suggestions appreciated (I’m just about technical enough to be considered a clear and present danger to myself and those around me ?? … here is the message text:
Warning: require(/home/ribserv/ribservices/wp-includes/date.php) [function.require]: failed to open stream: No such file or directory in /home/ribserv/ribservices/wp-settings.php on line 117

Fatal error: require() [function.require]: Failed opening required ‘/home/ribserv/ribservices/wp-includes/date.php’ (include_path=’.:/usr/local/share/pear:/usr/local/lib/php’) in /home/ribserv/ribservices/wp-settings.php on line 117