SD card suddenly stops responding to CMD0 (RESET/SPI)
If CMD0 fails, it's either the SPI/ SDIO pins are not connected properly (check your connections) or the controller is not actually working. In my case, the controller wasn't working. I tried using it in my Linux PC, even gparted was unable to detect it.
I'm still unsure why CMD9 failed, I have tried to contact the manufacturer, let's see what they respond to that.
And that answers my own question.