Have you changed your tree from public to private recently ?
Something similar happened to me a while ago, after I was forced to make my tree private for a while. Several people who had been invited to view the tree had not clicked on the email, but still got to look at my tree, because it was public. When I changed it to private, they lost their viewing rights.
All you can do is to delete the invite, and issue another one. Warn the other person that an invite is coming, because they often get lost as spam.