The most common reason hens are not laying is the molt. I know it is the wrong time of the year for the seasonal molt, but there are other things that could trigger one. After they have laid continuously for a long period, generally over a year, they can get worn out and may take a break. Yours were not laying quite that long but it may put them in a position where something might trigger a molt more easily than normal. Different things can trigger a molt out of season, possibly involving stress. A change in length of daylight where the day appears to get shorter to them can trigger one. It does not have to be something you did intentionally, a street light or security light shutting down could do it. From what you've described this is my top guess.
Another possible explanation is that they are hiding a nest from you. This happens a lot. They can be experts at hiding nests from predators, they did it well enough to survive in the wild before they were domesticated. You are probably easier than many of those predators, especially if they free range.
In NC you have many predators that eat eggs. Most leave traces like shells or wet spots but snakes, canines, and humans do not. If it is consistent it is not a snake. A snake eats eggs and then disappears for a few days while they digest the eggs, then come back for more. Coyotes and foxes would be more interested in your hens so probably not them but a dog may eat eggs and leave your hens alone. Does a dog have access? A human does not necessarily mean a thief, some people think this type of thing is a good practical joke. But this has gone on for months and has been too consistent for too long. And you have started getting that one hen's eggs. This doesn't feel right.
So what can you do? Mark an egg and leave it down there. If it disappears something is getting them. Or if you can lock them up in the coop/run if they free range or in the coop only if it is big enough and you start getting eggs then either they are hiding a nest or you locked out an egg eating predator.
To me this has all the signs that something triggered an out of season molt and all you can do is wait. But a hidden nest is possible.