If you've checked that she doesn't have any mites, then she's probably moulting (some of them do it in patches). I'd up the protein-level in her feed as she'll need it to grow new feathers. The others will probably follow suit. Their egg production is likely to drop off as they moult and grow new feathers, but when they've got their new plumage and the day length really lengthens and the weather gets warmer the egg laying rate will pick up again.
If she does have mites then a lot of poultry keepers use Frontline (puppy/kitten strength) on the back of the next only and treat all hens at the same time - I must add that as I understand it, it is not licensed for use on chickens in the UK at the present time and you may prefer to seek advice from a vet who specialises in poultry. Advice on withdrawing eggs when treating with Frontline is ambiguous - you may like to look on some poultry fora - there's info here http://chat.allotment.org.uk/index.php?topic=5250.0
When I kept poultry I found it helped to keep a large shallow container of dry sand in the henhouse during the winter, as the hens' usual dustbaths were too wet to use because of the weather. The dustbath and preening helps the chickens rid themselves of mites etc.
If the hens do have mites then you also need to treat a wooden henhouse thoroughly with a proprietory treatment to get rid of the infestation. 