If you reset the XG, this last has the default ip address and subnet with default admin, admin as username, password.

Here the starting guide:

https://docs.sophos.com/nsg/xg/hardware/sophos-quick-start-guide-xg-86(w)-rev1.pdf

Regards

Hi HASSAN BAKARI,

Are you able to ping the internal interface ip address of the firewall? When you try to access the WebAdmin are you getting 500 server error? If yes, try to reboot the firewall and try to access it.

If you have a physical access to the firewall, try with serial connection Sophos XG Firewall: How to setup a Serial connection with a console cable and see if you can access the firewall. 

Can you please confirm if you have factory restored the firewall or re-imaged it?

Thanks,