Several factors may cause this issue. Check the following in order.
1. Turn off Mute/Solo in Track Editor.
2. If the track or master fader in the Mixer is at the bottom, raise it to 0 dB.
3. Turn off all Mute/Solo settings in the Mixer.
4. Check that DYNAMICS in the control parameters and track automation are not set to zero.
5. An unauthorized voicebank will not produce sound. Select an authorized voicebank. You can check its authorization status in VOCALOID Authorizer.
6. Check that the correct audio device is selected in Preferences.
7. If you imported a VPR or VSQX file containing lyrics for a different language than the selected voicebank, sound may not play even though the lyrics are displayed. Enter appropriate lyrics again.