Sometimes a shock is necessary to even get their attention if they are frozen out of terror. When it is bad it lasts a while and the person can just shut down because they feel overwhelmed and unable to deal with the stressor at all.

Getting them to help with something less stressful is a good way to gradually bring them out of it. When they are communicative you can help show them how they can act in incremental ways to deal with the stressor. Help them overcome the feeling of helplessness.

It can be as simple as when someone sees something happening, they freeze up and do not attempt any action to prevent the incident. This is usually temporary and they will move to help almost immediately after.

The classic example is a soldier's first experience in actual combat; when they are directly under fire for the first time. You never know how they will react. Training can prevent the freeze and provide an instinctive reaction that will carry them through (such as charging an ambush), until they see that they can deal with it, and then they are fine. Some never get over that first experience and will lock up, hunker down and can't function.

Stress inoculation in training helps to prevent this. This is why military training is, and should be, stressful. If you can't handle someone in your face yelling at you how will you handle someone shooting at you? Things like rappelling, mountain climbing, parachuting, and other things a normal person would have trouble doing, all can help to build up the resistance to stress. The learning to respond to orders and practicing physical actions in response to certain stimuli, such as immediate action drills, helps a great deal.