Has something major changed? New coop, recently moved, major storm happen?? Odd that the whole flock would stop, unless something has upset them. Maybe a raccoon has been disturbing them at night?
Are you giving them the same feed as always? They need high protein layer feed to keep laying, that, and lots of light.
Has your coop bulb burned out? My lights come on at approx. 3.30 a.m.. With increasing natural daylight you wouldn't think they would go into molt, as well their age. Still pretty young birds, but if their light hours are shorter because of a burned out bulb, they may sense it's heading into fall.
Pretty frustrating I imagine.