When you zonk out, you fall asleep suddenly. You may have seen babies be wide awake one minute and then zonk out the next.
A college student who was awake all night writing a paper might zonk out, and a toddler who refused to take an afternoon nap will usually zonk out at bedtime. You can also use the informal phrase to describe what happens when someone faints or is knocked unconscious. The word zonk became a popular slang term for "hit hard" in the 1950s, probably because the word sounds like the action of banging someone over the head.