You can't hear the far end site. Far end site participants are seated too far away from the microphone Your volume is set too low The volume on the TV is set too low The microphone has become disconnected at the far end. Ask them to check their microphone connection and reconnect if necessary. Far end site can't hear you. Far end microphone is too close to the speakers.
Far end has their volume set too high.
Ask far end site to re-position their microphone further from the speakers. Ask far end to adjust their volume. The screen is blank. The monitor has been turned off. The videoconferencing system has been turned off.