You could try following the DFU upgrade procedure. It's possible that there's a problem with the USB-serial (CDC) part of the 8U2 firmware (that's used to upload sketches) but that the DFU bootloader on the 8U2 is still okay. When you put the 8U2 in DFU mode, it's running other firmware and appearing to the computer as a different kind of device. Worth trying at least.