The ordinary reasons hens pause
Egg laying runs on daylight, so the commonest reason a hen stops is the season: as days shorten, laying slows or pauses altogether, and many hens take a proper winter break before restarting as light returns. Moulting does the same — a hen growing a new set of feathers puts her protein into feathers, not eggs, and will look scruffy while she does it. Age matters too; laying peaks young and declines year by year. Add broodiness, where a hen parks herself on the nest determined to hatch, a fright from a predator visit, a new bird upsetting the flock order, or a quiet drop in feed quality, and most stopped layers are explained without any illness at all.
Check for the hidden nest first
Before concluding she has stopped, confirm she has actually stopped. Hens are talented at founding private nests — under the coop, in a hedge bottom, behind the feed bins — and a hen vanishing each morning and returning pleased with herself is often laying beautifully somewhere you have not looked. Watch where she goes after breakfast, check the run's corners and any sheltered spot she can squeeze into, and count eggs across the whole flock for a few days rather than judging from one nest box. Finding a warm clutch of ten eggs under a shrub answers the question and saves a vet bill.
The signs that change the picture
A hen who has stopped laying and is simply well — bright, eating, scratching, dust-bathing — is a management puzzle. A hen who has stopped and is unwell is a different case. See a vet promptly if she stands hunched with tail down or upright in a penguin posture, strains repeatedly, has a swollen or tight abdomen, passes discharge, breathes with effort, stops eating, or separates herself from the flock and goes quiet. Those signs can accompany problems in the laying tract, including an egg stuck on its way out, and laying-tract trouble deteriorates quickly in a small bird. Do not squeeze her abdomen or try to manipulate an egg yourself; keep her warm, quiet and separate, and get veterinary help.
What to do while you work it out
Run the management checks in order: confirm the flock's feed is a proper layers' ration and fresh, check water is clean and unfrozen or unslimed, look for red mite in the coop's cracks, which torment hens at night and dent laying, and weigh her if you can — a hen losing condition needs more investigation than one holding her weight. Give an ordinary pause a little time while she stays bright, but diarise what you find: when she last laid, what changed, what you checked. That short record is exactly what a vet will ask for if she does need to be seen.
Standing in the run at dusk, still unsure whether this is a moult or something worse, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.