website and dashboard blank white page after WordPress Update

There is one thing I forgot to say.

The first thing you should do is to open the Network tab in your browser's development tools and reload the page. Does it say status code 500? Then the code is crashing, and you could follow the links I posted earlier.

Does it say status code 200? That's a particular issue with WordPress that your website is configured to use a certain theme, but the theme files are missing. If that's the case, you could do the following:
  1. Open your website in a file manager and check the wp-contents/themes/ folder of your website. There should be at least one theme folder there, preferably the one of the theme you were using on your site.
  2. Go to the control panel, go to phpMyAdmin and open the database used by WordPress. In the wp_contents table, there is a row where option_name = template. This value should match the folder name of the theme you just saw. If it doesn't, you can edit the value to match the theme name.