Best answer
In almost every case it is one of four things, and the channel page tells you which one before you start digging into the flow.
The token needs re-consent. Meta expires or invalidates access after permission changes, a password reset, or an admin change on the Page. The channel shows as needing reconnection and no send succeeds until someone completes the OAuth flow again.
A permission was removed. Someone edited the Page roles or app permissions and the scope your automation needs is gone. Same symptom, different fix: re-authorize and grant the scope.
A platform block. Meta paused the channel over a policy signal. Sends stop, inbound messages still arrive and are saved, and the channel shows the block.
The flow was paused for you. When a channel becomes unusable, ChatinFlow pauses the automations bound to it rather than letting them fail one message at a time. This is the case people miss: they fix the channel and the flow is still paused, because fixing the channel and resuming the flow used to be two separate actions. Automations that the system paused now come back automatically when the channel recovers.
Start at Channels, not at the automation. If the channel is healthy and the flow is published, then look at the trigger: a keyword change is the next most common cause.