This plugin re-writes your .htaccess file. I had the same problem.
To fix it just edit the .htaccess file back to its original format. Your file should read as follows:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
