I lost all data on my iMac recently, including some 1500 photos. These photos were synced to the iPad from iPhoto on my iMac.
How can I transfer them back to my iMac since they are not in the camera roll on my iPad?
iExplorer is an app that does the trick. I just tested it, and it works.
So it's quite simple and straightforward. Launch the app, and select you iPad from the sources, there will be a Photos section, and everything should be there.
The same thing happened to me once, but I used Time Machine to backup. It must be terrifying, anyway, hope it helps!