Self answering and describing the hard way to resolve for the poor souls around who would face the issue.
The problem was that the UEFI flash got corrupted somehow, probably because of an untimely power failure. The result of the corruption "removed" some of the UEFI disk drivers.
During normal operation, the UEFI builds a chain of drivers the following way, taking an USB drive as example: USB port driver -> USB Mass Storage driver -> Generic disk driver -> Partition driver -> File System driver
In my case, the generic disk driver went away, and consequently all the file systems were not available anymore.
I used a Raspberry Pi and a level adapter (3.3V -> 1.8V) to physically re-flash the UEFI using BIOS 1.8 binary image on the board and did recover all my data.
The attached picture describes the quick and dirty setup I hacked during lunchtime:
For the bold ones:
The UEFI flash (Winbond W25Q64FW) is a 1.8V chip. Do not attempt to flash using cheap programmers that may only handle 3.3V.
I used a make-do level converter using a 74LVC08 for pins CS, MOSI and SCK. MISO is an output from the flash and the Raspberry Pi can use the 0-1.8V levels directly