Shes broody. Three options:
1. Try to break her using techniques described in other threads
2. Leave her alone, remove fake eggs from all the nest boxes, take all the laid eggs everyday and let her just naturally get over it.Take about 2-3 weeks.
3. Get her some fertilized eggs to set.
A hen that starts out being broody can stay that way her whole life. Blessing or curse depending on how you look at it.
Odds are the other hens have found somewhere else to lay and you have not found it yet,
Especially if they free range your yard.
If they free range, lock them up until they find a new spot to lay in the run or coop.
If they dont free range at all, looking in every corner of your coop and run, you might find eggs in the least expected spot.
Eggs can be outside for over a week and still be edible, so they might still be good if this just started.