Flash card formatting error in Nikon D800 resulting in non accessibility to photos

Hi there - I would appreciate any insights as to what may have happened today. This is my 8th year as a photographer and for the first time today lost all my images after a photoshoot.

Using a SanDisk Extreme III 4GB card (probably about 4 years old) in a Nikon D800. As always I deleted and formatted the card before the shoot, took photos all morning and everything was fine. Got back to my office and thought I would use for the first time a Lexar card reader instead of loading the photos through the camera. To my astonishment, a message came up on the computer (an iMac) saying the card wasn’t readable by the computer. Unplugged it and checked on my camera and suddenly its saying the card needs to be formatted! 

I had no way to get to the photo files so I ended up going back out and re-shooting the project. If anyone has any ideas I’d appreciate hearing them. Thank you. Really can’t have this happen again.