I think I need a bit more information.. Did you change UPGs, or are you just playing 'free panel'? Did you mute the parts by touching the top icon accidently in the main mixer page (perhaps trying to touch a chorus send knob?), have you got any UPGs that link to MIDI Sets that turn the keyboard parts into MIDI send only? Did you move the sliders down thinking they were in Drawbar Mode? Do you have SMFs that control the Keyboard Parts? Are they from another Roland keyboard (do they contain sys-ex?) that you tweaked to control the parts?

There are several ways to do what you describe. Just try to remember the last thing you did the next time it happens..