If you choose a layout with fewer or differently named slots, existing content in other slots won’t display in the new layout.
The content isn’t deleted, it just can’t be displayed unless the layout you choose has a slot with the same name.
Make sure you choose a layout with the same number of slots. If the layout you choose has the same number of slots, but some of your content still does not display, it’s likely the slots have different names. Contact the theme designer to resolve the discrepancy.