They have been doing great and then just overnight they shriveled up and broke off at soil line. I don't see any insects. This is only happening with the ones in pots.
This is likely slug damage. They come in the evening and there are no signs of them during the day.
Here is a link that will help you with slug control ideas.
https://www.gardeningknowhow.com/plant-problems/pests/insects/kill-garden-slugs.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/slugs-eating-potted-plants.htm
New leaves on my impatiens are smaller than old ones. Why is that? How can new growth of leaves be larger?
When your plants began to get leggy, you can cut them back to improve their shape and condition.
Cut them back to about half of their size at planting time.
Make the cuts about 1/4 inch above a set of leaves.
This will rejuvenate your plants for the later part of the growing season.
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/growing-impatiens-flowers.htm
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/impatiens-problems.htm
How to return my yellow impatiens back to green?
These articles should help you identify the problem:
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/impatiens-turning-yellow.htm
http://web.extension.illinois.edu/cfiv/homeowners/070828.html
I have had 15 hanging baskets in my backyard with impatiens for the last 20 years. Always beautiful! This year they are all being destroyed, time and time again. Some look like they've been sat on and some eaten on. I don't see any slugs in the pots ever. I have squirrels, but they have never before done that much damage. I've sprayed them with poison, squirrel repellent, used cayenne pepper, nothing seems to help.
Slugs could be some of the damage, and they are rarely seen, since they come in the night to devour plants.
You can sometimes see the silvery trail from them left behind.
Rodents sound like a likely pest with the damage that you mention.
This could be mice or even squirrels.
Here are some links with more information.
https://www.gardeningknowhow.com/plant-problems/pests/animals/garden-mouse-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/kill-garden-slugs.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/slugs-eating-potted-plants.htm
White undersides to the leaves.
Unfortunately, this is probably a disease called impatiens downy mildew. This article explains what to do:
Is there a way to get impatiens to come back year after year? Something special I would need to do? They are planted outside in my flower bed. Thank You, Stephanie Cobb
Impatiens are annuals in any location with frost, but you can regrow them from seed if you collect the seed at the end of the year for planting next year.
Impatiens produces seeds in little pods that "pop" and eject the seeds when they are mature, so to collect them efficiently you'll need to tie little cloth bags over each seedpod (or over the whole plant) and remove them once the seedpod naturally releases the seeds. Make sure the seeds are dry, then store them in a tight-lidded glass jar in the fridge.
Here's how to grow the seeds the next spring:
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/impatiens-seed-propagation.htm
All of the sudden, last year, the huge impatiens died. My neighbor said I should have cut the plants back in July. Two questions... 1) What did I do wrong that they died in my hanging baskets (which the garden center said I should keep moist by putting a half to full gallon of water on daily), and 2) Should they have been cut back?
Your impatiens plants may have been overwatered. Most plants don't need to be watered every day, except possibly during hot and dry summer conditions. To decide when to water, feel the surface of the soil and water thoroughly when it is dry. Also make sure your soil and the container are well drained.
It is not necessary to cut back impatiens, but you can cut them back to improve appearance if desired. There are other possibilities for why the plants may have died, like diseases, so it might not be anything you did.
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/impatiens-downy-mildew.htm
https://www.gardeningknowhow.com/ornamental/flowers/impatiens/growing-impatiens-flowers.htm