With some keyboards, there are certain rhythms that are set up in such a way where nothing will sound until you play a chord on the keyboard. Try playing a chord.
Here is a list of keyboards which have rhythms set up in this way:
CTK-2000, CTK-2080, CTK-2090, CTK-2100, CTK-2200, CTK-2300, CTK-2400, CTK-3000, CTK-3200, LK-220, LK-230: rhythms 140 and 142-150
CTK-4000, LK-270, WK-200, WK-210: rhythms 170 and 172-180
CTK-4200, CTK-4400, LK-280, WK-220, WK-225, WK-240, WK-245: rhythms 171-180
CDP-200R, CTK-5000, WK-500: rhythms 190 and 192-200
CDP-220R, CDP-230R: rhythms 191-200
CTK-6000, CTK-6200, CTK-6250, WK-6500, WK-6600: rhythms E:029-E:038
CTK-7000, CTK-7200, WK-7500, WK-7600: rhythms E:030-E:039
Some keyboards also have programmable user rhythms. If the selected user rhythm is empty, then this will also occur, in which case you should select another rhythm.