These links might help you. There are a lot of reasons they might stop laying. You might see something in these links that explains your problem.
Virginia Tech Stopped Laying
http://pubs.ext.vt.edu/2902/2902-1097/2902-1097.pdf
Florida Stopped Laying
http://edis.ifas.ufl.edu/pdffiles/ps/ps02900.pdf
When they all stop laying, my first thought is a molt. I'm not sure where you are in the world. If you are in the southern hemisophere, then the days are getting shorter and this is a likely explanation.
Maybe something is taking or eating the eggs. A lot of animals that eat eggs will only eat a few and will often leave signs behind. When they all disappear, I usually think of a dog or a human. There are plenty of other things that might eat eggs, such as snakes, possums, raccoons, rats, or chickens, but I'd expect some to not get eaten with them or that you would see signs.
Maybe they are hiding nests from you. That has happened to me before, but that has always been one or two chickens, not all of them. You might try locking them in the coop and run for a couple of days if you can. If eggs show up, that might mean that they are hiding nests or it might mean you have locked an egg eater out of the hen house. When I have one hiding a nest and I lock them up, the nest hider will pace the fence when it is time for her to lay an egg, desperately wanting out, but eventually she gives up and lays in the nest. It sometimes takes longer, but usually if I leave them locked up for three or four days, she starts laying in the right nest again. It really helps to find her hidden nest and take those eggs away though.
Good luck! This is not always easy to solve.