|It doesn't sound good, does it. I suspect a blight but here are some other things to consider. Is there only one side that is brown and is that brown side facing more intense direct sun (southern or western exposure) than the others, or reflected sun & heat off a hardscape surface such as driveway or wall? Was salt applied to a road surface, that splashed up on the plant? Is there any chance that it received overspray from herbicide or pesticide spraying in the neighborhood? Any chance the roots were damaged during transplant? Is the soil compacted, i.e., a vehicle driven nearby? Is there an obstruction that would prevent the roots from expanding? Was it overfertilized, which can burn roots and cause dieback? I would not recommend fertilizing at this point. Fertilizer "forces" a plant to grow, which can be stressful if it is already suffering from other problems. Be sure that the plant is receiving sufficient water, but not standing in overly moist soil, which can rot roots. If none of the above fits the problem, here's a publication to read:
It includes photos of blighted arborvitaes.
I hope this info helps.