Is their pen covered?  If so, is it a solid cover or wire?  They may be seeing the hawk flying around at times during the day.  In that case, hiding is the smart thing to do.  In an open top pen or free ranging, it's what keeps them alive.
If your pen is safely covered and you want to encourage them to use it, I would give them more cover inside the pen.  Find or make things for them to hide under.  That will make the pen feel safer, instead of exposed.  If it's only covered with wire, you could cover the part closest to the house with something solid, even a tarp, to make it feel safer and less exposed.  If your chickens trust you as their protector, then I'd also hang out in the pen for a bit, so they know they'll be safe there.  Act relaxed and talk in a calm, happy voice.  If you normally talk to your chickens, say whatever you normally do, in the tone of voice you normally use.  Definitely bring a favorite snack as a bribe and a diversion.
If their pen isn't covered, I would either cover it or leave them to come out on their own.  If you have an open pen and an actively attacking hawk, staying inside the coop is helping keep them alive.