What sourland said!
She may be sitting for long periods in the nest box, and clucking when you approach, and even pecking you if you try to lift her off? Is she insisting on sitting in the nest box at night when she should be roosting? If so, then she is broody which is a natural instinct and nothing wrong.
Do you want your hen to hatch some eggs or not?
If yes, fine, put some fake eggs (even golf balls or large round stones will do) under her for a few days to make sure that she is committed to sitting on them, before replacing these with real fertilised eggs. You might want to think about moving her to a quiet box of her own where she won't be bothered. You will need to let/take her off the nest once every day to eat, drink and poop for no more than about 20 mins each time. About 21 days after that, hey presto!
If not, you will need to break her of her broodiness. Remove the nest boxes for a few days, or remove her somewhere else and don't allow her access to warm nesty areas. The key is to cool down her tummy. Some suggest a dunking in cold water every day but I never had any luck with that and thought it a bit brutal! Try and keep her distracted with treats and away from the coop. Whenever you see her lying down, pick her up and get her on her feet every time. It should take 3 or 4 days to break the hormonal hold over her. If you don't, then her health will suffer as she will not eat or drink properly whilst broody, and if this goes on for a long time she will get weak and sick.
Hope that helps!
