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I have used for some time, and currently use Nikon DSLR cameras. I've never seen this happen. My experience has been that they are extremely reliable. However, there is something about the way it handles storage folders that can cause the perception that images have gone missing. It has settings for both storage and playback folders. If they are different, then what you see on playback will be different from the images that you are taking. There are also some other peculiarities about storage folders, but they are mostly minor points. I would check that first. You can see that in page 82 of the reference guide, which I have uploaded for you here:
In addition, just to make sure you have all the images from the sd card, I would take the SD card and put it into your computer and copy everything - all files AND folders to your computer. That way you can be certain if the pictures are there you will not lose them. If you don't have an SD card reader on your computer, you can get one inexpensively. Here is a link:
I would start with that... then what I would do is check the storage and playback folders. To make things simple, once you know you have the pictures, I would recommend formatting the card in the camera and letting it create the folders. Let me know if that helps. But the bot***** *****ne is - those pictures you took - if you didn't delete them - they should still be there, even if not exactly where you expect.