Question : Cisco Unity Voice mail Error message to user. Message no longer available

A user gets a light on his 7940 Cisco IP Phone. He goes to check his message and he gets a recording stating "this message is no longer available".
Any ideas what causes this?

Answer : Cisco Unity Voice mail Error message to user. Message no longer available

Does the message get to / stay in the users in box?  Is the message left as unread?