WordPress CSS/Theme gone after SSL got activated

Sounds like you switched over to HTTPS and your site assets aren’t loading. Here are a few things you can try.

• Update your site address by going to Admin > Settings > General and making sure you have https instead of just http in the URL fields

• Add define('FORCE_SSL_ADMIN', true); to wp-config.php

• Change any static links in the content to use https

• Update your .htaccess file to something similar to the below code

   <IfModule mod_rewrite.c>
       RewriteEngine On
       RewriteCond %{HTTPS} off
       RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
   </IfModule>
  

I would start with the first one, do it and then check the site before moving onto the next one. Then continue down the list.