Bag worms treatment
One of the best things you can do at this time of year would be to cut out and discard any of the visible bags. They contain the overwintering eggs of the insects. Dispose of the bags in the trash so that the insects do not hatch/emerge in your landscape in the spring.
You will have to wait and see if there is re-growth on your Arborvitae in the spring. If the bagworms have caused major damage and you do not see new growth in the spring, you will have to replace the tree.
Bacillus thuringiensis, often called Bt, is a natural bacterium you can apply to control young bagworms. Typically they are active from mid-May to early June.
Here is additional information on bagworm control and what you can do. https://extension.umd.edu/hgic/topics/bagworms-trees-and-shrubs