Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. Exactly what happens when you try to access the address book?
The email team is aware that some users who had previously accessed their address book at Plaxo are currently experiencing issues getting into their address book at this time. Please be assured that we are working with Plaxo to correct this interruption as quickly as possible. Thank you for your understanding.