An HDMI connection begins with a handshake in which both devices agree on resolution, refresh and copy protection. If that handshake fails, the screen stays black even though the cable and both devices are fine.
The classic symptom is a device that works after unplugging and replugging the cable, or after powering the devices on in a different order. That is a handshake being retried, not a fault being fixed.
Sound has its own path. When a soundbar sits between the source and the TV, or audio returns to the bar over the TV's return channel, one wrong setting produces perfect picture with no sound.
Automatic device control adds convenience and confusion: it can switch inputs, change volume and power devices on and off in ways that look like faults.