A Few suggestions, but I am no camera specialist.
Make sure you have another backup first of all pics and video clips on the card to CD or DVD then format the card.
If above doesn't work, try it in someone elses camera, if it works then this narrows it to be your camera.
Try to Reset the settings for you camera if this exhist's ?
Or Look in your camera menu options for Card/Memory storage options for any setting incorrect.
Borrow someone elses SD Card any size with pics on to see if your camera can read the card.
Give the Slot where you card goes a good blow incase of any dust
Buy a new SD Card
Post the make and model of you camera.
Didn't think I could think of this many ideas myself.