Answer: I can't tell from your description whether your plants have lost their bottom branches, or have just lost the leaves on the branches. If the branches are still there, you can encourage new leaf growth by pruning the tips back by a few inches; if the branches are gone, the plants may develop new branches along the trunk over a period of time. Let's hope they do so the bottoms of your shrubs won't remain bare.
Arborvitae will thicken up if you prune them back on an annual basis. Early spring is a good time to prune, just as new growth is beginning. You don't need to prune them back a great deal - 4-5" off the ends of all the branches should be enough to encourage healthy new growth.
Best wishes with your arborvitae.
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