My approach to diagnosing systems such as this is to connect to the console serial port (probably 57600,N,8,1, or possibly 115200) and see what U-Boot and (if it gets that far) Linux boot messages say.
But it’s not clear there’s an official path to doing that, and what you learn may not indicate a readily fixable cause, though it would certainly be interesting.
Generally the idea on systems such as this is that you do a “factory reset” via a button or command which wipes the writeable overlay filesystem, restoring to the state of the read-only system image. If that system image itself is somehow corrupted (though, how?) then installing a new version would probably take TFTP over wired Ethernet. If the bootloader itself is corrupted, that’s at the level of JTAG or replacing the flash chip.
The serial might help you “see what you are doing” while trying to invoke the factory reset, or otherwise identify the issue. But if a factory reset doesn’t fix it, it’s probably beyond end-user self-help.