You're not alone.
I'm in the atlanta area too. Our community has about 20 of them and all but 3 or 4 of them died back to the ground. I guess they couldn't take this past winter. Usually in zone 6 and 5 people say they perennialize and come back from the ground each year. I do see a number of mature ones around that look fine and totally leafed out. I'm curious to know if others can lend some experience or observations.

Our have purple blooms and are troubled by some fungus as well. We are in wait & see mode.

That is pretty normal for many crape myrtles. They will often do that when young. One of my smaller one did that this year. Remove the dead trunks and the tree should rebound and do fine unless there's something else wrong.

let the new shoots that are growing up from the roots keep growing. They will leafout and produce the food that your crape myrtles root system needs.

Then Next Spring only cut off the dry and brittle trunks and branches.

Once you get the number of trunks you want and of those being ones that can survive your winters; only then resume cutting all other sprouts that you do not want.

Thanks for the replies (even though I didn't start the message)
Is it pretty certain that this happened due to cold and not because of some other factor?
We went down to 9-10 degrees for morning lows a couple of times this past winter.
Whatever variety we have doesn't thrive.

I don't know how certain anyone could be, but it is very common for young crape myrtles to perform this way. I don't think there is any reason to suspect anything different unless there are signs of pests, disease, or physical damage.

How young is young? They've been in the ground for about 6-7 years.

I guess that would depend on the variety and the climate. If they are growing back from the rootstock, I would assume they will be alright unless I saw other signs of problems.

The fact that you are around Atlanta AND they are 6-7 years old, does make me start to wonder, but without further issues, I would still have to think normal dieback from a cold winter. I don't know what Atlanta's winter was like, but I know here it was a little colder than usual here in Knoxville (compared to other winters of this decade anyway).

Have you folks contacted any Cty Extension places yet around Atl? You may have some kind of blight circulating your area that's hitting crapes as well as other specific varieties. We had a blight come through the Raleigh area several years ago that specifically targetted Arborvitaes, but they recovered that next season.