You can help first by not adding to that stress, second by sympathizing with the situation, and third by reducing the stress so the person can move toward their usual level of functioning. Here are some tips from sometimes-nonverbal people for what to do if a friend goes nonverbal and what to do if a partner does. Some of these are relationship-specific: any time you're close with someone who has a chronic condition that complicates their life, it is polite to ask them how they want folks to respond when those complications crop up. Other tips are useful for emotional first aid in general, such as if you come upon the scene of a car crash and a random stranger is rocking or wailing in horror.
The worst thing about a crisis is usually not knowing what to do and therefore feeling helpless. It's a lot less scary and uncomfortable if you know some things to try. You can usually get a crisis down to an embarrassing nuisance.
EDIT 3/7/18 -- Below is a discussion of what to do if you tend to go nonverbal in a doctor's office.