You can undo recent changes then redo them if you want them back.
Undo the last action: Choose Edit > Undo (from the Edit menu at the top of your screen), or press Command-Z.
Redo the last action you undid: Choose Edit > Redo, or press Shift-Command-Z.
If you want to get rid of the changes you made to a presentation since you opened it, see View, copy, and restore an earlier version of a presentation instead.