You don't say where you are located, but sometimes shrinking hours of daylight will cause them to stop. It takes a certain amount of daylight to complete the egg cycle, and as the days get shorter, the eggs slow down or stop through the winter.
But most often when that happens with my hens it's worms. You can take a fecal sample to a vet and they can let you know for sure, but I just watch the hens and when I see one make a fresh little pile - I look at it. If there are little white dots, or tiny white things moving then you have worms.
It's not to hard to treat and my girls are usually laying again in a few days. Wazine is a wormer that you put in the water, check the label for dosage. Let them drink that water for one day and pitch it the next day. Throw the eggs away for 10 days (they may have traces of the Wazine in them you don't want them eaten.)
Then, after the 10 days move on to a "broad spectrum" wormer, I use Safeguard for cattle (it's a paste.) The dosage I was taught was a small pea size amount for Bantums and a large pea size amount for Large fowl. I administer this right in their mouth (tried hiding it in bread, but one hen loved it and would rip the bread out of other's mouths before they could eat it - resulting in too much wormer for her!!) Then throw eggs away for 10 more days.
After that you are good to go. Just keep an eye on the poo every now and then to know if it's time to do this again.
After a few times of using the Safeguard, it is recommended to use a different broad spectrum wormer so that they don't build a resistance to it and it then it stops being effective.
