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Try to use a card reader on your computer to see if the card is working. It is possible you have a bad memory card or a faulty SD socket on your camera. If the card is recognized on your computer and can be formatted on the computer, then the card is ok.
Try another memory card (different brand) in your camera to see if it will work. If 2 cards don't work, then I suggest you take your camera to a local shop or repair center and have the SD socket examined, cleaned or possibly repaired.