Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If you want the repair done for you, that depends upon where you are located, so please Reply To Expert, with your zip code or other locality indicator.
If you want to do the repair yourself, you would need to replace the keyswitch, in all likelihood (though perhaps the failure is elsewhere, too) - let me know if this is how you would like to try doing it.
OK, good, understood.
Here are the Yamaha Authorized Service Center for your PSR300's type of instrument, nearest to you (about 22 miles away):
B STREET MUSIC AND RECORDING
245 S. RAILROAD AVE.SAN MATEO CA 94401
Tel(###) ###-####br />Distance: 21.85 Miles
285 N AMPHLETT BLVDSAN MATEO CA 94401
If this completes the case, please rate my answer, so that this case may be closed. Thanks, ***** ***** luck. (Let me know if neither of them is in business, or if they can't help you, or if you need more advice with anything.)