While editing in Layout view, you may inadvertently delete the data frame by clicking the delete button (on Standard toolbar), or by selecting Delete from the Edit menu.
When in Layout view, the Delete command deletes any selected graphic elements. Because the data frame can be selected just like any other graphic element, it is easy to accidentally delete the data frame.
When editing in Layout view, lock the data frame by double clicking on it with the Graphics Select tool.
The dataframe in this image is locked. This disables the Delete command on the Standard toolbar as there are no longer any graphic elements selected.