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There are 3 likely causes of this fault:
Damaged pins on camera. If the camera takes a CF card, then you can visually inspect the pins. If it takes a different type of card, then a visual inspection is more difficult.
Corrupted cards, if you have access to a external card reader or a card reader in a computer use it to try and read the card, if the card is corrupt it probably won't read. Sometimes if the corruption isn't too severe the card can be formatted in the camera.
Note: Formatting will delete everything on the card. Camera Clinic also offers a data recovery service if you have lost data due to card corruptions.
The camera may have a fault with the circuit board causing a reading issue.