2GB Memory Stick PRO Duo suddenly shows less memory available in camera!

We have been using the same SanDisk 2GB Memory Stick PRO Duo card in a Sony digital camera for the past 2 years and have been able to take up to 500+ photos on it at “large” file size format.  Just recently after clearing off all stored photos from the card (via download onto a Mac) and putting it back into the camera, the card now says only 40 photo images are available to take on the card at “large” file format! Can someone explain why it has suffered this huge drop/loss in memory storage and how we can recover the memory capacity and fix this issue? Would be very grateful for any assistance and answers!  Thank you.