Site looks fine but wp-admin is not found

This site has worked perfectly until today: http://www.reierstadridehest.no/

The wordpress files are stored in http://www.reierstadridehest.no/wordpress/

But when I tried to upgrade to 2.8.5 by clicking the Automatic upgrade WordPress responded that it would not be possible. Fair enough, I thought – and planned on doing it manually.

But now I get an 404-page when trying to access /wp-admin/ or any other page. NB. The admin folder is 755. http://www.reierstadridehest.no/wordpress/wp-admin/

The .htaccess is:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

Please help. I usually resolve my WordPress-issues by googling, but now I can’t seem to find the right answer to this one.

Thank you.