"....no power at all and no usb connection" = no way to recover using a software solution unfortunately.

Unless janch's suggestion of removing the battery for a while re-livens it you can strip it for parts because it's dead. I get that you're unlikely to know the history of the device however if it's been cross-flashed without clearing non-vol memory there's a chance region 154 is poisoned, or wrong data has been written to region 5 which is the bootloader. The other possibility is simply that the flash chip has failed suddenly and dramatically.