If you haven't tried already, try rebooting the phone. Also, what OS version are you running. You may be able to reset your notifications in your settings..
were you using the Voice Control app on the phone when this happen.? could you explain how you where utilizing the device when it went into the MUTE Mode... the Voice Control feature is accessed and uses the MUTE button to make selections in the Voice Control App .. I wonder if the side button is/was slightly stuck ... and if it is working correctly now..