It would be nice if there was a warning that a person is going to delete all responses of there form..
I had a person right click and left click and all responses were gone. I think he selected the upper left hand corner cell.
He didn't know about the "redo" the could have save him loosing all response from form.
You can recover your responses by clicking on the History widget in the bottom right corner of the View Responses Tab. Then just copy and paste the responses deleted into the latest version of the table.
Hope this fixes your issue