1. Can you show your images on a camera's LCD and cannot see these images on an external computer, viewing your SD as a hard drive?
2. Or vice versa, maybe you can see these images on a PC but not on a camera LCD?
3. Try to export images from a camera with Google Picasa, Apple iPhoto or other image collecting software; sometimes, it can help.
4. Do you see your files/DCIM folder as "hidden" on your PC? (For Mac, it may require changing a Finder option to view hidden files).
5. Check the state of a small trigger on your SD card, which locks a card from writing into it. Unlock it if it was locked, and then try again.
Best regards.