Yamaha PSR-S900 buttons not working

[Miguel90]

I replaced the factory display of my Yamaha PSR-S900 with a 7-inch sliding camera monitor.
I had to disassemble the instrument three times because the display was not positioned correctly, and after the third assembly the function buttons were no longer working. Now the instrument turns on, the display lights up, but the other buttons are not responding and I can no longer select styles or sounds.
I have disassembled and reassembled several times with no change
What could be the problem? ribbon cable? or some kind of panel?

 

[Martaine2005]

Very likely a ribbon cable broken or any wire for that matter.
You will have to check each wire for continuity.
But having no buttons working, does give an indication of what wires/ribbons to check. From main board to the board with the buttons.

 

[Harald Kapp]

Simply repositioning the cable's connector may help, too. Especially if the "ribbon cable" is of the FCP type.