Does the card work on your computer (using USB card reader) ?
If you camera is old and memory card large, it could be that the card format isn't the right one. For example, a 5 year old camera might not support SDXC cards properly, and if the camera is 10 years old it might not support SDHC.
For now, you can try formatting the card in your camera (read the manual), but if that doesn't help then it's likely a compatibility issue or broken card.