Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Electronic devices can grow old, even if turned Off and stored. (If the storage conditions are damp, or hot, or occasionally below freezing, or varying in temperature, this aging can be more rapid or worse.) I'm sorry to say it, but your Yamaha keyboard may simply need repairing. Unless Yamaha service centers do not repair this old a model, this can be done.
As for where a Yamaha service center for keyboards of the type you have, would be located near you, I would have to know your zip code, to look the information up for you.
At the very least, if you called several of them and asked for an estimate on the cost of getting your keyboard working again, you would have some idea of how much repairs would cost.
I would tentatively diagnose (from the sound of it) either failure of electronic controls or logic circuits in the keyboard, or else severe dirtying of the function buttons to the extent that none of them work. Either fault would require repairs, though the latter might possibly cost a lot less.