Open the Address Book from the Inbox page. In the left pane hover your mouse pointer over "Address book". Does it show a number? If so, click on "Collected addresses", and then back on Address book again. Do the entries reappear? Do you have any groups set up? If so, are they empty as well?
After about 4 hours on the phone with Comcast and their English challenged technicians, they cannot restore my address book even though the email addresses resides somewhere on their system, not on your computer. Lesson learned, make a copy of your address book onto your local system.