site down, faulty upgrade?

At least i think thats the version im on now.

My site is https://www.iselin.co.uk

I did an upgrade the other day, then i installed one of the widgets, css facebook button? which i deleted strait after as i already had one which wokrd far better. i then left it. when i came back i got this msg:

Warning: require(/mounted-storage/home98a/sub003/sc23123-GWTG/iselin.co.uk/blogs/wp-includes/class-wp.php) [function.require]: failed to open stream: No such file or directory in /mounted-storage/home98a/sub003/sc23123-GWTG/iselin.co.uk/blogs/wp-settings.php on line 68

Fatal error: require() [function.require]: Failed opening required ‘/mounted-storage/home98a/sub003/sc23123-GWTG/iselin.co.uk/blogs/wp-includes/class-wp.php’ (include_path=’.:/usr/share/php5/’) in /mounted-storage/home98a/sub003/sc23123-GWTG/iselin.co.uk/blogs/wp-settings.php on line 68

I run my site but it was set up by tom daniel of dmstudioworks.co.uk
so he’s cot the codes etc. I need to know what is up with my site as i really need it to be up and running. please help me sort this out.

iselin