Hey, it would help knowing which device you're on. But here are some quick tips:
If the card is empty, plug it into a PC and reformat it. Sometimes, this little trick works magic. If you have files on the SD, be sure to copy them onto your PC first as a back up.
If that doesn't help, do a reflash - a clean installation (back up and restore your device to it's factory settings).
Lastly, if none of the above help, do this:
Download the Cyanogenmod 10.1 Nightly (Google it) and flash the device in CWM. Proceed with a reboot. Midnight, you'll most likely be presented with a "internal SD card was corrupted" message and it'll give you the option to format it. Do it. This should fix it.