Yamaha YPG-535 Keys Do Not Sound Correct, Some Not At All, etc

YPG-535 Trouble. Can someone post an answer.

all keys seem to sound out of tune, different sounds for each key and some keys do not sound at all...
Tried to Factory Reset This does not help at all...
Checked each Individual Function Setting according to Manual This does not help at all either.

Is there anything else I can do to get to sound as it used to sound.

I am beginner and do not have much experience at this kind troubleshooting.

Thank You.

Nuport.