If the feathers have been completely pulled out, new ones may grow.
Otherwise, they will just keep looking ratty until the next time the chickens molt. Then they will gradually lose all the old feathers and grow new ones.
Most adult chickens molt in the fall or winter, but they sometimes skip molting their first year, and some molt at odd times for various reasons. Chicks molt several times as they grow.
Of course, unless you change their behavior, they will just do the same thing to the new feathers too.
Things that sometimes help:
--more space
--things they can go around or behind or on top of, to get a bit away from each other
--things to do (dirt or compost or bedding to scratch in, plants to eat, etc)
--fewer roosters
--higher protein feed
Different flocks need different solutions, depending on what is actually causing the problem, and in your case I don't know.