Cannot log in after changing the www prefix per htaccess [closed]

Question

I have a website (http://koereskole-svendborg.dk/) - as default it did not have www in front of the url and I tested some different htaccess such the one below with no success. I did not change any setting in WordPress where it is set to not have www in front of the urls.

After testing the htaccess (frontpage worked but subpages got a 404 error) below I put in the original htaccess - see below. But now I cant login on my WordPress installation anymore - what do I do to login and to solve the htaccess issue?

RewriteEngine On
RewriteCond %{HTTP_HOST} ^www.koereskole-svendborg.dk [NC]
RewriteRule ^(.*)$ http://koereskole-svendborg.dk/$1 [R=301,nc]

Original:
# 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

Answer 1

Have you tried using the default .htaccess?

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