You need to format the card in the camera first. However, due to the age of the camera and card, you may need to replace the card with a new one.
The files already on the card can be recovered but it will take some recovery software to do it.
There are many excellent recovery programs on the market today. The one I use most is Photo Rescue available at the site below. It's not free but I've found it invaluable on many occasions. It is easy to use and very thorough.