From time to time one of your chickens may go broody. For some this maternal instinct is stronger than others and it can happen at any time. It is quite easy to spot because the broody hen will simply sit in the nesting box (or flower pot!) and refuse to budge. She may also make a peculiar growling noise if disturbed and become quite aggressive. However, unless your chicken has been near a rooster within the last 7 days the eggs will not be fertilised and will never hatch into chicks.
If you allow eggs to collect in the nest then there is more chance that a chicken will go broody, so try to collect them regularly. Even if you are collecting the eggs daily a chicken may still go broody. She can stay like this for 3-4 weeks (the incubation time for eggs is 21 days). It is not necessarily a problem but it will prevent your other chickens from laying in the nesting box and as broody hens eat less she may loose condition.
What you should do
You can remove her from the nest and block it using something like a ball or flowerpot so that she cannot get in. She may well try to peck you, so wear gloves, open up the egg port and lift her off. After a couple of days she should lose the urge to sit on the eggs and you can open up the nesting box again.
