Carefully prune back all the brown and dried stems and blooms, leaving only a small portion of the stem visible above the soil. Do not cut any stems that are still green.
Mix the plant food with water according to the plant food's label and water the petunia until the soil is soaked through.
Place the plant in a sunny spot and monitor the moisture of the soil daily. Keep the soil moist until the plant begins to show new growth.
Once the petunia has started growing again, it may be fertilized or transplanted to another container.