If you edit a page that is using the page builder and you deactivate this by changing the page editing to the standard Text Editor this will break all the formatting and the functionality of that page. In this scenario you need to re-enable the page builder.
You do this by editing the page and clicking on the Beaver Builder tab which will take you back to the page builder and then click done in the top right corner and then publish. See the video at the bottom of this page for a video demo of how this happens
Here is a video tutorial of how to re-enable the page builder on a page where its been disabled. Please make sure you have sound available.