The AI stopped replying

In the vast majority of cases it's a deliberate safeguard. Check, in order:

1. Did you take over the conversation? A manual message pauses automation for that contact — click "Resume automation";
2. Did the contact opt out? Opt-out active; only the contact can revert it;
3. Plan limit reached? See Settings → Billing — automations pause at the ceiling;
4. Is the channel in "Off" mode or missing an agent? Check the channel's Agent Builder.

The article When the AI goes quiet explains each safeguard in detail.