One scenario could be they were in the middle of a war game and were radio silent. This would include radars. I wouldn't think this is likely, but might be a possibility.
The two scenarios , loss of comms before the collision or radio silence would not explain why the crew of the Fitzgerald had to use Satellite phones to call for help way after the collision.
Of course, it is possible that one of the two scenarios is correct, AND they lost comms during the collision due to the flooding/damage from the collision/etc.