Warning and Fatal Error Message regarding class-wp

I accidently moved something in my server and now this message shows up when trying to access my site:

Warning: require(/homepages/35/d338917646/htdocs/wp-includes/class-wp.php) [function.require]: failed to open stream: No such file or directory in /homepages/35/d338917646/htdocs/wp-settings.php on line 68

Fatal error: require() [function.require]: Failed opening required ‘/homepages/35/d338917646/htdocs/wp-includes/class-wp.php’ (include_path=’.:/usr/lib/php5′) in /homepages/35/d338917646/htdocs/wp-settings.php on line 68

Does anyone know how to fix this? I’ve found a few similar problems on forums but they talk about using Windows (I’m on Mac) and get very complicated with coding which I’m terrified to try in case I cause more problems.

Can anyone explain what the problem is and if there’s a solution? Any help would be much apprecated