Sorry that you're having that issue, the Action History feature of the builder will only record the actions of the current session, once you exited or reload the builder those actions history will be gone.
Please try the Wordpress' Revisions feature instead.
What is: Revisions
To play it safe, please save your page as a Content Template first, before doing the revision.
If revision did not work still, then there is no other way but to rebuild those accidentally deleted section.