No Audio when playing back

I was recording vocals using Cubase 5, the first time I played back the recording in edit mode the vocals were not very loud although the settings were high. The next time I tried to play back the recording there was no vocals heard although the instrument track was rockin. This has never happen to me before and I am not good at figuring out the problem, since C5 has worked so well in the past. What should I do :question:

Check the channel output is assigned to the master buss., make sure the monitor is not switched on.