Did you by any chance happen to make a backup of your WordPress files and the database before updating? (if you don't actively make backups yourself then maybe with some luck your host does this automatically for you)
when you say WordPress 3.5.1, do you actually mean 3.5.1 or the latest version which is 3.9.1?
If you were updating from 3.5.1 to 3.9.1 there could have been a problem or conflict that occurred during the update that caused the fatal error. And as far as i know the only way you could recover from this is by using a old WordPress release (3.5.1) getting your WordPress active again and then deactivating all plugins and then try updating WordPress again first, and then your plugins (update before reactivating plugins). But this would entirely depend on if you have a working version of your database.
If you haven't made a backup of your WordPress website, now is a good time to use an FTP client to backup your WordPress files, and then login to your Hosting cPanel and make a backup of your MySQL database