Same thing happened with my 5D MK II. Most of the times, the problem is with the card. i.e., You might have browsed your card in some random PC or Mac (which might have virus).
What you need to do is, After you copy all the photos from the card to computer, please make sure that you format the memory card from the camera itself before you start shooting again. So that, this won't happen again.
To recover the pictures, I always follow a simple method. Mine is Windows 7 PC. So, I simply insert the memory card in my PC, and after the external drive opens up, I'll search for
*.* in the search bar. So that it would show all the files that were written in the memory card, irrespective of the nature of the file. (careful, do not select the affected files) Copy only the pictures you want to recover and paste them in a different location.
[Unlike other methods, like photo recovery software, this method is not guaranteed. Yet, you could give it a try. Most of the times, it might work]