Trouble shooting a hedge of Arborvitae may take a few back-and-forth Q & A with you. To start with, check out the article from OSU Extension about the conditions required for Arborvitae to thrive, and some of the common reasons for decline. Which of these do you guess might be true in your case? https://today.oregonstate.edu/news/arborvitae-stands-tall-low-maintenance-hedge
We have had drought years recently, causing many evergreens to decline. Describe your supplemental summer watering routine. A friend of mine lost a hedge plant because a sprinkler head was clogged. She didn't know until it was too late to save the tree.
The other thing that catches my attention in the photos you've provided are fairly young (probably rapidly growing) full size conifers across the fence. Those will have wide root systems competing for water and nutrients in the soil for your hedge. Sometimes plants will co-exist well for years before one claims too much light, water, or soil and the other plant declines.
Is the location particularly boggy in the winter? If so, the root damage might not be reversible.
Tell a bit more about the growing conditions.