First of all... don't worry. You haven't lost anything, you have just selected a layout with a smaller number of cells. That means that the elements that are left over are “in reserve”. You can see them again in the list of elements in the slide properties panel marked with an alert icon. From this list, you can move these elements to the slide cell of your choice.
You can also view it again by selecting a layout of more elements. If you still want a less elements option and you have free cells or prefer to do without some other resource, you can use the move element functionality, or you can also delete it.
You can move the element to a visible position in your slide or to another slide.