Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Tell me what the model is, please.
For many models of Yamaha, turning On the MIDI Out will mute the sound (except through the MIDI Out port), so check to make sure you aren't in MIDI mode by some mischance.
Otherwise (but not certainly, before knowing what your model # ***** Yamaha is) you might have a problem or failure in the amplifier circuitry of your synthesizer. It might require repair work be done. But first, once I know the model #, I can have you try one or two things that might bring it back into operating condition.
So, let me know, please. Thanks.