Sounds very similar to what happened to me some months ago. I own a 2GB SanDisk Ultra II SD card and I am still using this card with my S3 camera. When I bought this memory card, I could read/write without any problems from the built-in card reader of my laptop. But suddenly something happened and the card reader no longer recognizes this card. I have another 1GB card, which the reader accesses successfully. The strange thing is that this same reader can access my girfriend's SD card which is 2GB SanDisk Extreme III.
I tried to reinstall the card reader's drivers but this was of no help. So I gave up and bought an "el-cheapo" USB card reader from eBay for 1 pound. The advantage is that this small USB card reader can be used as USB flash drive (if the SD card is not full with photos
)