Macbook air cannot access files on SD after being used on PC
I have a macbook air mountain lion. I used to have no problem importing pictures from my SD card to my macbook. Recently, I let my friend borrow my SD card to drag pictures from it onto his PC. Now when I insert the SD card into my macbook, I cannot access anything in it. How can I convert it so that my macbook can access the files? Thanks.
MacBook Air, OS X Mountain Lion (10.8.3)