Did you just turn TVGuardian OFF? If so, foul language was probably detected and a mute was triggered just as you selected OFF. Go back into the TVGuardian App and turn the TVGuardian mode to STRICT. Once the sound comes back on, you may turn the TVGuardian mode back to OFF.
If the above did not restore the audio,
Your TV or audio receiver may have interpreted the lost audio stream as a "not in use" connection, and automatically disabled the audio.
Turn OFF your audio receiver for a few seconds and turn it back ON. If this doesn't do it, try turning off your TV for a few seconds.
If this continues to be an issue, you might want to turn off the HDMI-CEC feature on your TV and/or audio receiver.