Digital keyboards vary in size (some have the same amount of keys as a full-sized piano while others have a few octaves less), but they all are designed to replicate the functionality of a piano while giving you greater flexibility in moving the equipment around. If, however, your digital piano keyboard is not producing correct audio, you need to look over the hardware to determine what is causing the equipment problem.
Check the power connection running from the digital piano keyboard to the electrical outlet. If the equipment is not completely hooked up to the power outlet, the piano is not going to turn on. Likewise, check the cable connection running from the piano to any speaker. If the cable is not inserted completely, the speaker is not going to receive the audio sound.
Spray compressed air in-between the keys. If dirt and other material falls between the keys they may not push down completely, preventing you from playing the specific note.
Disconnect the digital piano keyboard from the electrical outlet. Leave it unplugged for a few minutes as it resets the internal settings on the keyboard, correcting settings such as the wrong audio type or octave playback.
Unplug any headphones currently connected to the digital piano. If headphones are connected, you aren't going to hear any audio from the keyboard when played.