Voicemail isn't working

When I try to log-into my voicemail using my home phone, I get a message that "this mailbox is currently unavailable."  I am getting notifications that voicemail messages are there, but can't access them. If I dial *99, I get the same error message.  If I log into the account online to try to check voicemail there, it says "An error occurred while processing this request. Please sign out and try again later."  How do I fix this and access my voicemail messages?