If you've got a genuine SanDisk and the problem exists, it's been reported by various others. If I were you, I would try the following:
use another SD reader with a different interface, and/or even a different PC (or Mac) to try reading it to rule out defective reader or interface (unlikely)
with the card as-is (i.e. don't format it) try a commercial purpose-built recovery tool like Lexar ImageRescue (try as many as you can). This might not work since these tools work on formatted or corrupted cards (you might be lucky). If that fails, as a probably last resort, try:
format the card in the camera using the Yes, not overwrite (Yes, will just erases the fille allocation table & not the images themselves - this will force a recoverable situation based on no file allocation table), and then try using the above image recovery tool.
don't do any of the above, and contact Leica customer support & maybe they'll take on the recovery of your SD images as part of an investigation.
Please make your choice, without holding me responsible
Additionally, (hopefully after you get back your images) I would also suggest a card (write/read) test as I outline in my blog (in sig below). Note this will completelly erase your card. This will prove the card is not defective, and indicate another issue.
Good luck - but sorry to say, I am not optimistic given the extend of the damage.