SD Card Woes

Hi all. I'm a professional photographer and have a have a SanDisk SD card. In the middle of shooting, I got an error message on my Canon EOS 60D that the card wasn't reading. Naturally, I had a backup SD, so I swapped it out. When I went to plug the OG back into my computer, it didn't show up. Plugged it in on another computer, didn't show up there either. Other SD card worked fine, no issues at all, but this one wouldn't read. I flipped the switch on the side of the SD card to see if it was an image protection issue, and it comes back that ALL OF THE IMAGES WERE GONE (but the SD card is now reading and recognized). Can someone point me into the right direction as to what the issue is and how I can recover those images - since I assume they are still on the card, just not showing up?