I have a Kurzweil Re-210 digital piano that has no sound and the display shows---. I have tried the MiDi/Pref button and nothing happens and have contacted multiple repair people and no one has an answer.
According to the owner's manual, the instruments resets settings on powerup so it should be playable regardless of what was set under MIDI/pref.
That the display shows nothing but (- - -) indicates that the problem most likely isn't in audio output (DAC, amplifier, speakers...), but an internal problem.
The service manual seems to be easily found by google. Here's the top result:
There you'll find simple instructions for the diagnostics test. Diagnostics mode is entered by pressing "DEMO" and "MIDI/Pref" buttons simultaneously after power up.
If that doesn't do anything, you'll probably need to open the chassis and check that all cables are intact and connected, and whether there's physical damage to the circuit boards.