The wide variety of issues that could be preventing your media card from functioning properly are as follows: The media card...
* is not supported by this camera (e.g. supported type, capacity, etc.)
* is inserted in a slanted or improper position and may need re-positioning
* terminals are dirty and require cleaning with a dry cloth or cotton swab
Check your device and see if these may pertain to your specific issue. If not, the media card may be damaged and be in need of replacement.