Why did my potted mums turn brown and brittle? And are they dead?

Did you keep the soil moist?Or the opposite was there a lot of rain and the flowers were saturated?

It sounds as though they didn't get enough water...I would try soaking them (or putting them in a tub with 2-3 inches of water, whatever level it takes to cover the holes in the bottom of the pot). They may stillness be salvagable

May be salvable, but sound dad to me. They are typically potted in soil that is not very good. So watering is needed every few days.

Add one table spoo of hydrogen peroxide to a gallon of water . Water the plant with it. Soon the leaves will gradually be revive

Sometimes mums get little mites that do kill them. My first plant with a mum did. I did not know about them. Perhaps they also will die for lack of water. You could water it and see if there is still life in the roots. I brought back two such plants this fall for the same reason. Good luck!

as a door nail