Have you seen this?
https://docs.duet3d.com/en/User_manual/Troubleshooting/Duet_wont_respond
Please try erasing and reflashing the firmware once more, make sure you go through all the steps shown here.
https://docs.duet3d.com/en/User_manual/RepRapFirmware/Updating_firmware#all-other-duet-boards