Hi Geri. It will depend on what the chair is made out of. If the chair is wicker you would really have to have a professional refinish it. Some wooden/metal chairs you are able to cut a new piece of wood the same size and attach it.
Need a little more info as to what type of chair.
If you can remove the seat, then you just duplicate it with similar materials.
It all depends on the materials and style of your chair. You can purchase webbing from home improvement stores. You can recover a seat with outdoor fabric. If it’s cane or wicker you can replace it with jute or other cording or rope.