If it were my board and out of warranty, I would remove the 3.3V regulator, feed 3.3V to it through the Z probe connector, and try to communicate with it through USB. If that didn't work I would use the Erase jumper to see if the Bossa port appears, and upload the firmware if it did. If I was able to get fit working like that over USB and Ethernet, then I would replace the 3.3V regulator and the 2 chips that are powered from 5V.