Is it possible you have egg eaters?
Same thing happened to me once, we had a sharp drop in eggs all of a sudden.
I would never believe it since I thought they were getting everything they need, free range all day, purina layena, yogurt and endless water etc.
I could not understand why they would do turn to egg eaters unless they were bored or something even with free ranging.
I would see a hen on the next box and when I would check later there would be no eggs in the box.
Then I found a piece of an egg and knew I had a problem.
I staked out the coop and watched for anyone with eggs on their beaks since I could never actually catch one doing it.
Found two that way.
I seperated them from the flock, fed them extra protein and calcium, put golf balls and round rocks in the boxes so they would try and peck them instead and maybe they would find it unappealing, anything that was recommended here on the forum to break them of.
I thought they were all set and returned them to the coop.
My husband walked into the coop one day after that and caught the two of them actually eating eggs.
They went to auction.
It may be anything ,sometimes its just a change in their routine, their surroundings, or their food etc. that causes stress and then a drop in egg production but if you are not getting any eggs at all I seriously suspect egg eaters.