I have the Manual for the KN2000 and these are the remedies it suggests for "no sound is produced when the keys are pressed".
The MAIN VOLUME is at the minimum setting. Adjust the volume with the MAIN VOLUME control (sometimes we are so worried we overlook the simple solution - we have also had no sound and its been because the expression pedal has been knocked, and when children push buttons [I'm not sure how old your son is but I know what ours is like] or change things we don't think about it).
It also mentions that maybe volumes for selected parts are set to the minimum levels (check on the screen) but if you were using the keyboard the day before and it was fine, it is not likely that this is the problem. Have you used a Midi file in the keyboard as you sometimes need to reset after this?
From the Manual - If the buttons, keys etc malfunction - turn off the POWER button once, then turn it on again. If this procedure is not sucessful, turn off the POWER button once. Then, while pressing the three lower left buttons in the RHYTHM GROUP section (8 beat 2, Jazz rock & soul, and Dance Pop) at the same time, turn the POWER button again. (This will return all settings, functions and memories to their factory-preset status). I think this is probably the same as initialising the keyboard through Memory & Control - did you have "Total Keyboard" highlighted when you initialised?
I read your comment re turning Midi on or off - this should not make any difference to whether the keyboard will play.
When you push the Demo button do you get any sound?
What were you doing before you turned off the keyboard the previous day?
If none of the above fixes your problem please write again. Another member once told me that the only "dumb question" is the one we don't ask (and I've asked plenty) so keep writing and I'm sure someone will solve your problem.
Thank you so much for giving me the opportunity to try to help someone else on the Forum - I have received so much assistance from many members and I would really like to able to know enough to help a Fellow member.
Good luc!