Try doing a low-level format on the card by using the camera. (The card may have become corrupted.)
In the camera display, select the Setup menu and choose Format, Format Memory Card, or something similar.
If this doesn't work, tell us the brand and model, whether this camera ever worked, did you just replace the SD card with this one, just do a format (on the computer) on the card, anything else that is pertinent.

It is more important to know the camera you are using, and full details of the actual problem, please.