There is an Undo button for maps but not one for Layouts.
There should be an undo button specifically for actions in Layouts.
I just deleted a data frame by mistake. It would have been great to undo that action. If I hit the Undo button it only undoes actions I did in the Map (which in my case was removing layers from the map).
Hopefully I'm wrong and I don't know where the Undo button specifically for layouts is???