Did you check under Settings > Call settings > Voicemail service??? This should be set to Google Voice.
If it is set to Google Voice... I'd try using your PC's browser and go to
https://www.google.com/voice, then under the Settings gear symbol in the upper right, click on Settings, if it's active you might need to deactive it, then click "Activate Voicemail" link.. this will give you a "*" followed by some numbers to active your number on your phone again... you need to open your phones dialer and put in the * and numbers, then click the green phone button to talk... I'm sure this is explained to you when you click the "Activate Voicemail" link.