is she setting on eggs?
or an empty nest waiting for a shipment of chicks?
have you gotten her out of the nest to get up, eat and drink and exercise the legs?
replaced the nest box bedding with clean bedding while she's been sitting?
was the bedding you just replaced dirty or damp?
what did you replace it with?
this hen either has Scaley leg Mite, or skin sores from irritant from the bedding..or as ddawn posted..similar to bed sores.
if she is not sitting on any eggs, try and wash the legs with very diluted cat shampoo or unscented baby shampoo..rinse well, and dry completely before returning her to the coop.
Examine the legs closely and figure out if it is sores or mites..
sitting hens are vulnerable to mites...so it wouldn't be a bad idea to give her a dust for mites.
you can treat the legs with neosporin, or a vet ointment called Panalog..(name brand)..generic is Animax..which treats both bacterial and fungal skin problems.
also as ddawn posted..the hen must get up and exercise the legs..don't put food and water right nest to her..but some steps away..and make her get up and feed and "go"..
keep an eye on the droppings to make sure they are normal, and not runny. (note that sitting hens will make big droppings from sitting so long..but they should be a normal color and not watery)
check the breast bone and see if it's prominent (indicates loss of weight)
what all are you feeding her?
she could probably use some vitamins and some extra protein, such as chopped egg and a little plain active culture yogurt.