It sounds like your FAT (File Allocation Table) is corrupted. Do not try to take any more photos with it until you recover the ones you're already taken. In other words, don't write to the card (don't use any utilities that write to it, don't delete any photos, and don't take any more photos). Otherwise, you'll risk overwriting some of the images on it.
Here's a thread explaining how to recover any images that haven't been overwritten yet using free software for that purpose:
After you recover them (and make sure you can see the larger photos, not just the thumbnails), then reformat the card using your camera's menu choice for Format (not a PC). That insures you always start out with a fresh FAT (File Allocation Table), which acts as an index to let your camera know where photos are stored), just the way the camera wants it, since it's performing the Format. I do that every time I reuse a card (format it using the camera's menu choice for format) to reduce the chance of a corrupted FAT that may not be obvious until you start having issues.