Yes, they were probably zapped by the most recent cold spells on Thursday-Saturday nights, when overnight temperatures dropped as low as the mid 20s or in early March when they skidded to the low 20s.

Maintain the soil as evenly moist -not dry but not soggy either- as you can & with 2-4" of mulch around the drip line to minimize any further temperature extremes to the roots from cold temps or hot temps in the summer. The plants should should develop new foliage "soon" but probably not as fast as you may want to (2-4 weeks). If you have not done so yet, delay fertilizing until after your average date of last frost; for example, for Blairsville, that would be around the last week in May.

This frost would not kill the plants but it leaves the foliage looking quite ugly. Some hydrangeas that bloom on old wood now have invisible flower buds so resist the temptation to clear the foliage away by pruning stems. It is ok to cut the petiole string that connects a leaf to the stem but delay any plans to prune a stem until you can confirm it is really dead and the chance of late frosts has really passed, especially in the mountains. If the flower buds were already opening like some of mine are (they look like tiny broccoli heads), they may have been zapped (when killed, they brown out; when only slightly injured, part of the bud browns out).

In the future, you can try to water deeply before the cold temperatures arrive, mulch and use heavy blankets or frost cloth to see if they can minimize injuries. Winter protection can also help but it is hard and maybe impractical to apply it as late as April. Note: make sure that any 'heavy blankets' will not weight enough that they may break stems.

Speaking of stems, wait until May to prune any stems that do not leaf out. When I was living in Atlanta, I would wait until mid May or later... depending on the weather for each year,,, if we were still hitting the 30s overnight. However, from what I remember, overnight temps in North Georgia should be mostly above 42F now and if this holds for a long enough period, the risk of another frost would be low... but not impossible.

This incident may reduce bloomage this Spring season but you should get bloomage from the new growth that ES produces this year.

The frost may have zapped some of the old wood invisible flower buds typically located at the ends of the hydrangea stems. Hopefully, few flower buds where killed and hopefully the "back up" flower buds are viable and will open instead. Annabelle-type hydrangeas would not be affected by last Friday's frost as it is too early to be developing flower buds but a rare, stray late frost later May might.

CMs will also leaf out again after a pause. They do not need as much water though so you can just water when you feel that the soil is dry.

Be careful out there: today is a very low relative humidity day in several areas of Georgia so there are high fire danger conditions.

I was going to ask pretty much this exact question! My 3 Endless Summer hydrangeas are looking really sad since the frost a couple days ago. I'm in upstate South Carolina

Try to maintain the soil well mulched and as evenly moist as you can. I use drip and some optional elbow grease when very hot. They may end up developing new growth from the base to replace any dead stems but allow any leftover leaves to continue producing food for the roots.