How to Unformat a Memory Card

"Just now, I inserted my memory card to my Android phone and connected it to my computer because I wanted to transfer some data from hard disk to the memory card. However, Windows 7 system asks me to format the memory card when I attempted to access the card. What will happen if I format the card? How to unformat the memory card and get back lost files if I format the card to fix not formatted error? What should I do to access files on the card without formatting?"