Hello:

If they just stopped working-As "they were working, I changed nothing, then they stopped"- I would guess something changed in your system.

If you are running Time Machine on your Mac I would roll back to a date where everything was working. I would try and reinstall the USB MIDI Driver and see if that helps as well but... you have provided zero info about what software you are using, what you are trying to accomplish, etc. It helps to have a full story.

Also Yamaha has support resources. Here are contact sites for the US, EU and Japan:

musicproduction@yamaha.com
Additional support for US customers can be found at
http://usa.yamaha.com/support/contact/
Additional support for EU customers can be found at
https://de.yamaha.com/de/support/contacts/
日本のお客様はこちらもご利用ください
http://jp.yamaha.com/support/music-production