I believe it is going into a dormant state. I have maidenhair fern that is growing wild on my property (I live on a river) so there is the humidity, and my zone is 8b, and I notice that in the winter this beautiful fern gets alot of dead fronds on it and looks pretty beat up, but come spring, the baby new fronds start peeking out. Since yours is in a pot, I would say slipping the pot into a large plastic bag, leaving it open, will keep the moisture level up high enough, but not enough to rot the crown of the fern. Of course, this fern likes it semi-shady to shady, so I wouldn't be leaving it in a sunny window.
Good luck to you!
firefly