Did you overwater?
Both petunias and pansies can fall victims to disease ....the latter to something I experienced few years back, viz., pansy sickness. Could also be simply aphids. Or vine weevils.....lightly tug the plants. If they pull out easily minus roots its vine weevil damage
I would cut back everything....overwatered or not....aphids or not. Then push your finger into the soil and see if it's wet....I assume drainage holes ? I would spray remaining foliage with soap spray or insecticide. The next day I would water and feed.
with petunias and pansies I would cut back hard now anyway. They often run out of steam about now so I cut back, water and feed for quick regrowth of foliage and flowers