Have you tried uninstalling the device driver, unplugging it, and re-installing and accepting the driver it wants to install?
Failing that working, you could use the sketch here:
Note the wiring carefully. You will need SD card board.
You could read (save to SD card) the existing contents of the 8U chip, just in case.
Then you could reflash it with the 8U firmware (I don't think I have one but it should be on the web site here somewhere). That may or may not work. Possibly they used different pins on the 8U to stop you doing this. Or possibly the only reason they didn't put the "official" firmware on it was for copyright reasons.
My Mega doesn't have the ISCP pins (the header is just solder pads) so I can't easily read it.