No idea myself, but did you try these suggestions already?
https://www.google.fi/?gws_rd=ssl#q=recover+deleted+files+iphone I checked the first one, it suggests using certain iPhone app for the task, apparently you need to connect the iPhone to a PC (or Mac?) as well.
The first important rule whenever you are thinking of trying to recoved deleted or lost data, is stop using the device so that you don't rewrite over the deleted data sections in the filesystem. I presume this applies also to iOS.
i guess it also depends how exactly did you lose the data. Did you delete some files with some kind of file manager, or did you reset the whole phone to factory defaults (and lose all your data in the process) etc.? A bit like I presume it matters a lot on PC whether you simply deleted some files, or if you e.g. deleted the whole partition where the files were.