Hi,
I have exaclty the same problem. I was using it fine and out of the blue when connecting the arduino mega 2560 r3 no changes to the device mamanger.
I have also plugged the arduino into a pc with no arduino drivers installed and It wont even come up with device unknown etc.
The arduino still play's the program that was last uploaded to it.
I have since tried to get it to dfu mode without luck.
Most of the instructions are not for the Rev 3 so the diagrams are different.
If anyone knows exactly how to get the Rev 3 one in dfu please advise.
Please help someone.