Welcome to the forums, Burto!
Let me take a look at the manual to see what functions are available on that model, but I'll tell you right now that my first inclination is to suspect that the "Local Control" got turned off by your son. I'm not sure if that setting gets saved when you turn off the power, or gets reset to a default value (which should be "on"), but that's what I'd check first if I were you.
EDIT: Okay, I'm looking at the User's Guide right now, and the Local Control parameter is function number 14 in the Function menu (see pages E-4 and E-36). Basically, if that parameter gets set to "off," it will keep the keyboard from making any sounds when you play it. That might seem like a strange thing to want to do, but sometimes you might want to do that so the keyboard can play sounds in response to MIDI data that you're transmitting to it from a computer or other device, without the danger of making "unwanted" sounds if you were to accidentally hit some of the keys while the external device is controlling the keyboard.