Freezing up or being unable to move or respond is called what reaction?

Escape reaction.

Explanation:

The anxiety / fear reaction is part of a defensive behavioral system built by evolution to face dangers. This system detects a hazard and produces responses that aim to increase the likelihood of survival in a hazardous situation. There are some basic reactions that the body presents in a dangerous situation. The best-known "fight or flight" response formula actually consists of four basic directions of behavioral responses: fight, flight, freezing, or fainting.

Let's imagine that we come across a dangerous animal. If our brain interprets that we can face the animal, our body organizes itself to face or intimidate the animal. But if the animal seems too big or dangerous to our resources, the response it organizes is escape. If there is no possibility of escape, a possible response is paralysis, freezing, as a strategy of trying to go unnoticed. If the attack is rated as imminent, a faint response may arise to try to pacify the enemy or faint to protect us from the pain of the attack.