Try the memory card on another camera,
* if it works (or you use another known to work card and confirm the problem stays the same) the problem is with your card reader:
* ensure the camera's switch is not stuck and the inside of the slot is clean, you can follow Flying Dutchman's tips from [post|305830|this post|new_window=true] (if it helped you solve your problem, please feel free to leave a feedback to the author)
* If it doesn't, it could be it's corrupted or the filesystem may be incompatible, backing up everything on it and formatting it with your camera should solve the problem
Hopefully you found this useful,
cheers