When the nervous system perceives a threat that cannot be fought and cannot be escaped — when both the fight response and the flight response have been assessed and found insufficient, unavailable, or already exhausted — it does the third most intelligent thing it knows how to do. It shuts down.
The freeze response is the oldest of all survival mechanisms in the vertebrate nervous system — older than fight, older than flight. It is the strategy of the prey animal that goes limp when caught, of the child who learned that neither fighting back nor running away was an available option, of the person whose nervous system encountered something so overwhelming that the only remaining strategy was to become as small, as still, and as imperceptible as possible until it passed.
What is particularly important to understand about the freeze response is that it is not depression, not laziness, not a lack of motivation or willpower. It is the nervous system's most ancient protective strategy, doing exactly what it was designed to do. The person who cannot get out of bed, who cannot make the decision, who stares at the screen without being able to begin — is not failing. Their nervous system is protecting them from something it has assessed as overwhelming.