J'ai vu ça sur mac os x hints:
I was fooling around with a vCard importer application when I somehow damaged my Address Book database. All of my contacts were gone, and even importing the backup that I created minutes ago didn't work. Address Book just sat there and did nothing, displaying an empty screen for minutes, so I was sure that I couldn't import the backup that I created before. Somewhere I found a hint stating that it would, however, rebuild its database if you deleted it. The database is the file named AddressBook-v22.abcddb, located in ~/Library/Application Support/AddressBook.
So I closed Address Book, deleted everything from that directory, and copied, from the package contents of my last Address Book backup, only the folders Images and Metadata, plus the file MailRecents-v4.abcdmr. Then I started Address Book and, voila!, all contacts were back where they belong.
[robg adds: I would suggest moving, rather than deleting, the files -- just in case. Once you're sure things are working again, then trash the files.]