When a device fails to boot, SunxiTools becomes your lifeline. Put the device into FEL mode, then use sunxi-fel to write a new bootloader or full firmware image directly to NAND flash or SD card.
The SunxiTools team addresses this with . Every verified release comes with a checksum (SHA-256) posted on a separate, static website. Before running the installer, compute the hash: sunxitools windows verified