Your card may have become corrupted, there are good free utilities out there to save such files from corrupted memory cards, try DIR (Digital Image Recovery) that you can use with a computer card reader if the computer does not recognize your connected camera as an external hard drive. 

If you were able to recover your files, format the card in the camera, that should solve your problem using the card.

If the card cannot be recognized by the computer or formatting the card doesn't help, you may have a defective memory card, try another one.

If the card works otherwise but is still not recognized by the camera you may have a defective memory card connector or other camera motherboard problem, sorry.

Hope this helped...