Moment
Sometimes, you don't respond to people not because you're ignoring them, but because you're simply exhausted. You're not avoiding the conversation, you're just drained from the thought of having one.
You've got no energy to explain yourself, no energy to justify your silence, and no energy to care about the other person's expectations. You're not even thinking about the person on the other end of the conversation, you're just trying to survive the moment.