They've gone broody. That is normal, brooding behavior. They collect a clutch of eggs (which you are taking) then stop laying and set on the eggs.
Typical brood lasts 3 to 5 weeks. Some refuse to stop unless they either hatch a chick or you break them of the brood (which can be difficult in some breeds like Silkies).
You have several choices:
1. Let nature takes it course. They will stop brooding eventually and go back to laying.
2. Give them fertile eggs and build your flock with new chicks.
3. Purchase feed store chicks and let them foster them.
4. Attempt to break the brood by either repeatedly taking them off the nest, placing them far away in the yard, even blocking the nest. You could also put them in a broody buster cage which allows airflow to their belly to cool off their hormones, literally.
As for me...I let it take its natural course unless the bird is beginning to suffer from interminable brooding....but usually I place fertile eggs and use a good brood. I've blocked hens out of the nest, taking them out repeatedly giving treats. I personally don't prefer the broody buster cage.
LofMc