Final upshot: Once I got home, I connected the Nikon to my laptop and discovered the folder in question was no longer concealed. I opened it in the usual way and downloaded all the images.

But the question remains: Why did this happen in the first place? I found that the folder was holding 497 images. Could it be that each folder has a maximum capacity of 500 images and after it reaches that limit, that it opens a new folder? The Nikon D500 manual makes no mention of that. In addition, once that 500 putative limit was reached, why wasn't I able to review them on the camera? I had to wait until I could download them to my computer to review them. If that's the case, it's something that Nikon needs to remedy. Does any of this sound plausible? Thanks again to those of you who offered suggestions.

After shooting about 300 photos last night of an event (thank goodness, it was a social, and not a paying job), I found that my camera had somehow switched folders and only the folder to which it switched could be accessed later on. There were only 18 shots on that one. The original folder would not open. I have several folders still filled with images, but enough room for another 600 images on the camera. In looking through each of these folders, there was one which looked decidedly different:

Instead of showing a half filled folder which is what the graphic normally creates, the original folder showed upper and lower section of a folder along with a message: "Selected folder number cannot be used. Choose another folder number." In front of this message there was a large exclamation point.

Near the end of the photo shoot, I had replaced my battery. Also, during the shoot, at times I was using the high-speed option (10 frames/second), but I'm sure I had allowed all the photos to transfer from buffer to card before switching the camera off to replace the battery. Can this apparently damaged folder be opened? Suggestions hugely welcome!