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It sounds to me as if the camera has a failure in the operation of its memory card slot.
The card was working last Saturday, cards work in the computer's memory card slot. But they don't work in the camera's memory card slot.
So the memory card is the right size for the camera's memory card slot, but doesn't work in it any more. Was the memory card written to, while in the PC's card slot? if not, then the camera's memory card slot is indeed 'broken' in some fashion.
The questions that remain are:
1. when you look into the memory card slot on the camera, with no card in it, do you see any pins bent, loose debris in the slot, or other unusual thing? there are several metal pins or terminals inside the memory card slot, that should be in line with each other, and free of dirt or corrosion. If they are dirty or not aligned with each other, you have a problem there.
2. how old is the camera? measured from date of purchase.