In the old GUI, go to "Reporting" => "Partition" and explore the graphs for the Pool containing the missing folders/dataset. If data has been lost, you should be able to see the Green and Red area change for the Pool and the corresponding datasets (You will need to press the zoom-in and zoom-out icons and the left arrow on the graph to get a better view of the span) . If the change is substential. and you will be able to see the Min and Max value, based on that you can tell if you actually lost data and when.