Chances are they are pooped from our long, hot, dry summer. You can try cutting them back, watering, and fertilizing and see what they do, though it's getting late for the plant to have time to recover before cold weather shows up. Depending on the kind of petunias you have, there's a chance they may reseed for you. I only grow mine in pots (petunia integrifolia) and they seed around all over the place. They are usually the earliest thing to emerge in the spring, they bloom like mad till hot weather, then die. THEN...in the late summer, they come up again and bloom until frost.