There are several possible reasons. Follow these steps to find a solution:

(1) On the Track Editor, turn off the "Mute" or "Solo" setting.

(2) In the Mixer window, if the track fader or master fader is set to the smallest value, move the fader up to the 0 dB position.

(3) In the Mixer window, if the "gain" knob is at the bottom-left position, turn it back to the "0" position (which is the 12 o'clock position).

(4) In the Mixer window, clear all the "Mute" and "Solo" settings.

(5) If you have selected an expired singer, no sound is output. Select a singer that has been activated successfully.
You can view activated singers by selecting "Help" and then "About VOCALOID4 Editor...".

(6) In the "Preferences" window, select the "Audio Settings" tab to check if the correct audio device is selected.

(7) Open the standard Windows sound mixer, and make sure that VOCALOID4 Editor and its destination device are not muted or set to a low volume level.