I've had "dead" various Yamaha keyboards before. Buttons not working (none of 'em) could be something wrong with the E-BUS (I2C bus). Could be interconnect. Could be power (+5V). My MO6 had an Ebus problem. I cleaned up the connections and still had an issue. Isolated the issue and did some reflow I believe on one of the I2C to GPIO parts (there are a bunch scattered around). I've got equipment to look at this stuff.

Start with a cleaning of the interconnects. I use Kimwipes & alcohol or no-clean flux off. Typically with a small brush (metal "straw like" handle) with the bristles cut short. This one is already cut down (and two ends - I just have bristles out one):

There's a video from Brother printers that shows cleaning the same type cables where they use a paper towel & alcohol. Same kind of idea.

Some people use erasers - but that can leave a residue.

Wear an anti-static wristband properly grounded (and tested) if you can. Don't zap your equipment worse than it may already be.

That's probably step 1. Usually I would look over what's inside actually first to see the lay of the land. Look for delamination (a high-temperature event causing melting / discoloration). Can't really give a sufficient primer if this isn't in your wheelhouse - not sure who I'm dealing with here. Since there's a possibility of making things worse - decide if you're the right one to "fix" it or if a service tech should be paid to do this.