When you are making the WordPress upgrade, there are a few problems with the integration can be raised. If you see any, please use the checklist below.
Firstly you should check the WordPress dir. In most of the cases new .htaccess is written here. The content can be like this:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>
# END WordPress
This code have to be removed. If there is not any additional lines - it's better to remove the .htaccess fully.