If your server is on IIS  and   running WordPress and experiencing an issue where navigating to /wp-admin results in either a blank page or a 404 error, you may need an additional rule in your .htaccess file. Generally you’ll find that, assuming you’re set up properly, you can reach the admin section by appending /index.php to the URL, but who wants to do that every time?

The rule necessary tells the standard permalink rules to ignore the wp-admin directory when performing rewrites. Simply add the following condition to your .htaccess file:

Here is the .htacess file just upload it into wordpress root folder
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_URI} !^/(wp-admin|wp-admin/.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress