I think to get the factory reset on that keyboard, you hold down the 'ENTER' button, while turning on the power. You then get a dialogue, to guide you thru a reset proceedure. This will clear the memory, also...so you will need to reload any custom sounds you may have created.
I would also suggest you just check out the 'channel mute' buttons ...to see if you may have disabled any of the channels. The button under each channel number (1-16), will toggle the sound on/off. Another thing to check is the volume setting for the channel (mixer function)...you may have the setting turned down (0= off 255= loud).
I hope this will help you!