Note: Use the latest version of the Divi theme.

You can know about the latest version here. If you are using an older version of Divi, you can learn how to update Divi here. 

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We will divide this troubleshooting into two major categories: newly added and existing pages/posts.

If you notice that The Divi Builder is absent, that could be from the fact that is disabled from Theme Options -> Builder -> Post Type Integration:

You can simply enable Divi Builder for Page/post and other post types and re-check on the page.

If the above settings are correct and Divi Builder is already enabled, you can follow the same troubleshooting steps below:

There could be five major reasons why the Divi Builder is missing from existing pages/posts (newly added ones could fall into these categories too):
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​     Plugin conflict
​     Child Theme conflict

Corrupt Divi Theme
​     The Default WordPress Editor is used
​     Server configurations

Divi has a feature to check the site in Safe mode.

You can go to WordPress Dashboard > Divi > Support Center and enable the Safe Mode:

Note: Enabling Safe Mode will temporarily disable customizations and plugins that may be causing problems with your Elegant Themes product. This includes all Plugins, Child Themes, and Custom Code added to your Theme Options integration areas. These items are only disabled for your current active user session so your visitors will not be disrupted. Enabling Safe Mode makes it easy to figure out what is causing problems on your website by identifying or eliminating third-party plugins and code as potential causes.

After enabling the Safe mode, check the Divi Builder again. If Divi Builder shows fine on Safe mode, this confirms either a plugin, the active child theme or some customizations are causing the issue.

Disable the Safe mode and perform the following tests:

Often when The Divi Builder goes missing on existent pages/posts, it happens after a theme/plugin update or a plugin installation. In that case, inspecting the page where Divi Builder is absent by clicking F12 in the Chrome browser, will help you to get an idea about this. Usually, a plugin conflict looks like this:

To check if indeed that's the case, deactivate all the plugins with no exceptions and refresh that page (you can do that from Chrome -> Incognito mode just to be sure no browser caching is involved). If The Divi Builder is present after doing that, it means a plugin is conflicting and you can activate them one by one, refreshing the page after each activation to see which one is the culprit.
With over 55.000 plugins in the repository, you can find an alternative plugin which does the same job at www.wordpress.org/plugins

It can happen just like a plugin may be conflicting, a child theme could prevent it from showing up. So simply switching to the parent Divi theme will show you if that's the case. To activate the Parent theme, go to WordPress Dashboard > Appearance > Themes and activate Divi Parent theme. Then re-check the page if you can see the Divi Builder or not.

If a child theme is conflicting, you need either to troubleshoot it by checking the codes from it, or contact its developer.

It can happen sometimes that the last time Divi theme got updated, a few files might have got corrupted. You can simply re-install the Divi theme following this tutorial here

then re-check if Divi Builder appears or not.

There are cases when The WordPress Default Editor is clicked in a page/post, thus preventing The Divi Builder to be present. So simply activate it to use it:

If you are using WordPress Gutenberg then the pages in the backend will appear like this:

When the Gutenberg editor is active, we can't use the Backend Divi Builder but you can only use the front-end Visual Builder.

If you want to enable Backend Builder, you can go to WordPress Dashboard > Divi > Theme Options > Builder > Advance and enable these 2 options as shown in the screenshot below:

2.5 Server Configurations:

In some rare cases, the server on which WordPress is installed could prevent The Divi Builder to be loaded in pages/posts.
This case resembles with the one when a plugin is conflicting and you can inspect that page too and check for the status codes errors in the console.
If the errors are coming from the 5xx Server errors, it means something on the server is preventing the builder to load and the hosting company should be able to check the server logs and fix that. The most common errors of this type happen with WordPress, no matter what themes/plugins are used and the error is present on the platform files:

Following the above steps should help to find the cause of Divi Builder missing and help you get it back.