It started, bizarrely, with my CANON 650D freezing for a while. I tried turning it off and it wouldn't - I suspect I did this too soon after taking a photo and the card was still saving the data. 

Now when I try using my camera it flashes a message saying there's something wrong with the SD card (which is 16gb). I've tried formatting it to the camera, but it was unable to. 

Help!