My companion stopped replying. What happened?

The chat tells you which of these it is, and your message is never lost. Most often it is the daily allowance on your AI access being used up (it resets at midnight UTC), or a paid model you do not have the credits for. Chats are also paused for a short while from time to time, and the chat says so when that is what happened. If the model itself failed, you get Retry and Switch model options right where the reply would have been. If the internet connection was lost while your AI character was replying, the part of the reply that arrived is kept, and you can tap Continue.