It happens sometimes, that Muffin Builder stops showing up in the back-end while editing already existing pages or creating a new ones. There may be 2 reasons for this:
1. Muffin Builder visibility
Please go to Betheme > Theme Options > Global > Advanced > Layout and check, if the option responsible for displaying the Muffin Builder, has not been disabled by accident. This option allows to disable builder for specific Roles or everyone.
2. Page Options
While editing or creating new post or portfolio item, click on 3 vertical dots in the upper right corner, then choose Options from the drop-down list and when the popup will appear, please make sure that Page Options option is checked.