A layout (and the content you've placed in it) may be overwritten when you accidentally click on another layout while working on a page.
To revert back to the previous version of your page:
- Click the Undo button on the upper left hand side of your editor page
- Press Command/CTRL + Z on your keyboard
However, the Undo button will only work if you haven’t saved and closed the page. If you have exited the editor page or refreshed it, all changes to your design will be permanent. At this stage, there is no way to retrieve overwritten designs.