Try:
- switching to the default theme by renaming your current theme's folder inside wp-content/themes and adding "-old" to the end of the folder name using FTP or whatever file management application your host provides.
- resetting the plugins folder by FTP or phpMyAdmin.
- re-uploading all files & folders - except the wp-content folder - from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.
- running the upgrade manually via wp-admin/upgrade.php