Hi there,
Firstly - are they in molt?? Do they look tatty, are feathers falling out, generally looking blah? After the first year or so, chickens will molt, this effectively ends laying for some time since molting is a heavy drain on resources. If they are molting, you will need to 'up' the protein in the diet to enable them to recover more quickly.
Alternatively, any new stress for them? New additions, change of coop/surroundings? Chickens don't like change and it can put them off laying for a while. We are also coming up to winter and laying does slow down some.
Have you checked them for parasites? lice, mites etc. Do they need worming? Are they eating/drinking ok? What does their poop look like? normal?
These are a few suggestions for you to look into, let us know what you find :/