Voicemail wants me to set up, but it was already set up.

First of all, I'm frantic. My daughter was murdered a year ago, and I've kept her phone number active so I can hear her voice on her voicemail. I just called to listen to her voicemail, and it is telling me to set it up. Her voice is gone. Why on earth did this happen?! Is there a way to get it back? Please tell me it was some random glitch today and will back working as usual tomorrow!