My visual voicemail disappeared when i upgraded from iPhone-4 to iphone-6. When I tap the voicemail icon it calls voicemail and prompts me to enter my password instead of showing visual voicemail.
Try entering your voicemail password when it asks for it. If you can't remember it, Verizon support will help with that. they helped me once with that. I called the 1-800 number. Good luck.