This might answer your question : http://forums.adobe.com/message/3600835#3600835
I think the main thing you have to guard against is people deleting their forms. Once a form is deleted it is gone (incl. the data). We do warn the user but looks like accidental deletions still happen sometimes.
So how does one access backed up form data? I may have had an accidental delete - not the forms just rows in the form.
If you only deleted rows of data you can try to look at the "History"
In the View Responses Tab, look in the bottom right corner for the clock icon. This will bring up the history of the response table. You can look at older versions and caopy and paste the deleted row in the current version of the response table.
hope this helps