Plan B: try a different SD card, preferably Sandisk. Size doesn't metter. And try to format the card with this handy tool I always recommend for sd cards:
https://www.sdcard.org/downloads/
There are also sd rescue images that try to restore the system partition. But for this to work you need to know the installed firmware version number.
Plan C is opening the device and looking for some ISP pins (in system programming, direct access to flash memory) but that's beyond our scope I'm afraid...