i am not able to access the contents of my SD card in nokia lumia 520. i can only see that there is a SD card in my phone but i could not access the contents of card.
The images and videos are automatically shown in the Photo hub or in the Music hub. The other files can be viewed using a file manager application like this
Hope the last answer (@Matteo) is not what you're expecting. Hope you don't have an option to format too. Just format the card with other Lumia device and try again.