Updated to 4.3.1 and website shut down

There is a problem in your header file which is stopping the theme from displaying.

View Source: the last line that prints is:
<header id=”masthead” class=”site-header clearfix”>

This means that whatever the next line is will be a wordpress/php function that is failing to execute.

Check your error_log file for more detailed information about what is causing the problem.

Where do I find my error_log? I only ever login through the site and some of my widgets are missing as well.

Your hosting providers will be able to point you towards your website’s error logs, if they do create them. Some don’t create logs.

If that is the case, even if you find the error_log, chances are you’re not going to be able to fix the problem.

This is a bad PHP function in your template – the error_log will no doubt direct you to header.php and the function that exists right below the code that generates:
<header id=”masthead” class=”site-header clearfix”>

You realistically have 3 options:
1. Learn PHP
2. Get a web developer to look at it for you
3. Change themes

How would I change the theme since my host doesn’t keep logs?

You should be able to log in still, and switch themes in Dashboard > Appearance > Themes.