If your arborvitae has brown patches in it but is still mostly green, it is possible to save the plant. Make sure it is getting enough water and sunlight and check the pH balance of the soil. \
The healthier the environment the plant is in, the healthier the plant will be. Browning areas in the plant usually indicate that it isn't getting enough water, though this can happen if the plant is not receiving enough sunlight. \
Prune the brown areas of the plant, do what you can to make its growing environment as healthy as possible and with luck, the arborvitae will recover and turn green again. \