Hello, my name is Rusty. We can turn off the transpose function to see if that clears it up, I want you to try this first and tell me the results. While holding down [DEMO/SONG] and [PIANO/VOICE], press the C3 key.
Are you sure it was the transpose that caused the problem? Could it have been bumped? When that set of keys is out, it normally means a ribbon came loose inside the piano, or the transpose was the straw that broke the camel's back in your tone generator.
It's not going to have much value without all of the keys working, due to its age. If the reset (turning it off) isn't working, there's a problem with your hardware. Unplug the keyboard, and wait one full minute. Try it again, this will reset the electronics, and hopefully jump start the bad electronics.
It's actually more like a disconnect. The tone generator works with 7 notes in different octaves. The keys are attached to the tone generator in sections. Click here to see how to take apart a Clavinova. Look at the connection to that set of keys, and make sure everything is tight, and nothing is burned, or white powder around the capacitors where those keys connect to the tone generator.