There is a big difference in the way deciduous and coniferous plants grow. Deciduous plants produce new growth from latent buds all along the branches.  Pruning a branch will allow buds below the cut to grow and fill in the pruned areas.  If you cut off the top of a maple tree, for example, it will form a dense network of new branches competing to become the dominant (leader) stem.

Coniferous plants form buds only either at the tips or along the green growth part of the branches.  The older wood with no green growth, closer to the trunk, has no buds to regrow.  If you prune a conifer branch into the old wood beyond  the green growth, that branch will not put out new growth or grow.  This can leave a blank spot in the side of the plant.  If cut off the top of a conifer like your arborvitae the only new growth you will have will come from the green horizontal branches.

On your arborvitae trees the upright shoots from the green parts of the horizontal branches will eventually grow out and up vertically to fill in the top open area of the arborvitae, but it will take several seasons to see .  The cut-off top will not regrow.  Conifers put out new growth yearly, so next spring you should see the top start to fill in.  Keep in mind that if the horizontal branches are typical and only have green growth on the outer foot or so of the branch, the middle of the cleared spot may never fill in.  Old wood will never form new growth.

Your arborvitae should remain healthy because you've left enough green growth to provide food for the tree.  One adage about fertilizing is that fertilizer is for growth, not healing.  It is better to wait until time has healed the wounds and the tree is better able to make use of increased elemental nutrients.

Arborvitae can grow up to 60 feet tall, depending on the cultivar.  Often a new homeowner finds an overgrown arborvitae hedge out of proportion to the site that is very difficult to control.  Arborvitae form a beautiful luxuriant green privacy fence, but need to be pruned regularly (every year or two from planting) to maintain a manageable size and shape.  Once arborvitae have grown too tall it's often more effective to remove them and either plant something more appropriate to the space, or embark on a program of regular pruning and shaping.