If a single Thuja ‘Green Giant’ in a row of healthy plants dies, it is generally a sign of root rot. This sometimes happens in just one section of the tree, which is an indication that only one root has died, but eventually they all will. Unfortunately, the soil in the area can retain the fungus that causes the rot, and that can impact a replacement plant. You can try to plant a new tree, which will grow quickly if it remains healthy.