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.
- assuming that you are/were using custom permalinks, creating & uploading a new .htaccess file containing:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress- running the upgrade manually via wp-admin/upgrade.php