trouble shooting 101
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FACTORY RESET
the first place I'd start is with a factory reset. Here's the process:
Hold [Enter] and power the unit on. The display will show the version number, then rotate the knob once to the right and the screen will display “Reset Parameter”. Press [Enter] and it will take a few seconds and will then complete and the unit can be rebooted.
INPUT/OUTPUT
One of the notorious problems with rack mounted devices is that it's so dark back there where all the connections are made.
1) Get a flashlight and make sure the mic is plugged into the right place
2) make sure the speakers or mixer is connected to the OUTPUT L and OUTPUT R jacks
3) make sure the cables work in other devices
4) test the headphone jack with headphones. This gives you a 2nd way to see if the device is getting a signal
HARDWARE
it's possible that the unit itself has a problem. The Voiceworks is old enough that it's probably no longer supported by TC Helicon, but I guess you can ask. You're more likely to get results by googling a local electronics repair person. These devices are cool enough to merit spending some money to repair it, especially since you got it for free. There are currently 3 of them for sale on ebay, one of which is being advertised for $749 USD
SAVED MIDI SETTING
These devices are popular among people who like to use MIDI to control it. It saves the current setting by default, so if the previous owner used MIDI to fade the volume to zero (but neglected to restore the volume after the song ended) its possible that the internal volume is still set to zero. If so, you can fix it by sending a value of 127 to continuous controller #87 (master output)