Hello, my name is***** is not a sign of major damage. There are velocity sensitive pads that can corrode which cause a single key to go out, or play loud or soft at random. They are a maintenance item, and parts are still available.
You want to remove the screws from the bottom of the piano. This will allow the top to flip up so you can access the contact strip.
When you remove the bottom screws, the panel will flip up, and you'll be able to see the innards of the keys. Press a key, see the pad it hits on the back of the key? You can order a new contact strip from Yamaha. Call(###) ###-#### Monday through Friday, from 7:30AM - 5:00PM Pacific Time (closed 12PM - 1PM).