There are several possible reasons. Please confirm following things.
(1) Please release Mute/Solo settings on the track editor.
(2) If channel fader or master fader of the mixer screen is set to the bottom value, set it back to 0dB.
(3) If gain of the mixer screen is set to the most left down position, please set it back to 0 position (it is the position of 12 0'clock).
(4) Please release Mute/Solo settings on the mixer screen.
(5) If you are trying to use "expired" Voice Bank as a singer, it won't give you any sound. Please change it to the Voice Bank which activation is valid. You can confirm which Voice Bank is activated on [Help] → [About VOCALOID3 Editor].
(6) Please check if the correct device is selected on the audio device settings on the preference menu.
(7) If [Trace the Past Event] is unchecked, the sound is only output when it is played back from the beginning. Please check it.