Your card may require a reformat. Once you are sure that the contents have been saved or can sbe erased forever, place it back in the camera, turn on the camera and format it.
Press Menu Ok/SET-UP/FORMAT then select OK. Your SD card is now good as new(empty and ready to store photos).
If however you cannot format the SD card using your camera, then turn off again the camera, take out the SD card then format in a Windows machine.
What? You have a Linux computer instead? For this later case, use the powerful command gparted. This tool should be installed but use extreme care when using this tool. For it is possible to format your hard disk or the storage device with your operating system.You must select the proper device, knowing the memory capacity can help! Next, you may have to create a partition, then afterwards, you can format it with Windows FAT 32 for large capacity SD cards.
After successfully doing this, reinsert card in your camera and attempt to format it again. You should be able to do this, otherwise it is time to say goodbye to the card! Replace it with a new one for this unfortunate case.