You might have to use a DMM (Digital Multimeter) and test if there is any voltage present on the keys.
Normally with most electronic keyboards a voltage is passed through the operated key(s) to the CPU which then selects which sound to play depending on which input lead to the CPU it is on. As none of the keys work it seems as though the power rail for this particular section of the keyboard may be the problem.
Unfortunately I cannot find a service manual for your keyboard to be more helpful.
If there is no voltage present on the keys you may have to go back to the power supply board in the keyboard and find all the different power rails that it has to determine which one is not working. As it is only low voltage (9VDC) it is safe for you personally to test, i.e it is not life threatening as it would be if it was working on mains power, BUT do not short out or 'bridge' across any connections with the meter's test leads as you can still cause component damage anyway.