the pictures could be corrupted if you are getting errors trying to open them. this could be caused by issues from the camera side or possibly with the card.
You say you did change batteries. When the battery was low did you try shooting pictures until the battery was completely dead? If you are writing data to flash memory and the card looses power it can corrupt data stored on the device. It is very important not to try to get one more picture when you are getting low battery messages from your camera.
You can try using a data recovery software and see if the images are recoverable however if the data is corrupted then it is possible that recovery would only recover corrupted data.
sandisk provide rescue pro with their professional line of cards. If you have one of the versions that does not come with the software you would have to buy it. You could also look for free software on the internet.
SanDisk rescue pro offer
http://www.lc-tech.com/pc/sandisk-rescuepro-and-rescuepro-deluxe/
