Have you downloaded all your photos? If so, then the simplest fix is to format the SD card in your camera (don't format it in a card reader - that can lead to incompatibilities).
If you haven't downloaded all your photos, then you need to consider using recovery software. You'll need a card reader for your computer, and recovery software - I like Sandisk's Rescue Pro, but there are lots of choices out there.
As a tip for the future - I recommend that you download your photos from the card to your computer when you get home from taking photos, then put the card back in your camera and format it. Don't use the card as long term storage. Also, get into the habit of backing up your photos regularly. Burn them to DVD, or copy them to external hard drives. If you don't have at least three copies, then you WILL lose them.