I have approximately 200 Arborvitae trees approximately 18 feet high. There are 2-3 right in the middle of an extensive row that have recently turned brown. To the right and left they are perfectly green. Any suggestions? Thank you!

While not being local to your area, would suspect environmental factors impacted those evergreens that have failed. While they are all the same species or cultivar, individual plants have developed root systems unique to themselves. In a stressful season, summers or winters, those with a poor root system exhibit higher levels of stress, including death. Unless the main branches of the Arbs remain viable, the entire plant could be dead. when you remove them for replacement you can see what kind of a root system they had. Another possibility would be if extensive trunk damage occurred over the winter by rodents or rabbits.