Indeed. Asked and answered daily.
It is not unusual for the Outlook Address Book to "lose track" of the
connection to its Contacts Folder when you move or import your PST or update
your Outlook version or OS. Use the following steps to reset the connection,
depending on your Outlook version. Note that in some instances you may
actually have to remove the Outlook Address Book completely from your
Profile, close Outlook, and then re-add it before you can get it to work.

Outlook 2000, Corp/Workgroup:
Go to Tools > Services. Make sure the Outlook Address Book service is
listed. If not, add it. Next, R click on the Contact folder or folders you
want the OAB to display, choose "Properties", go to the Outlook Address Book
tab and check the "Show this folder as an E-mail address book" box. You may
also need to go to Tools > Options > Addressing Tab and choose to show your
Contacts folder.

Outlook 2002:
Go to Tools | E-mail accounts, select "View or change existing directories
or address books", and click OK. If you don't see the OAB in the Directories
and Address Books list, click the Back button, then select "Add a new
directory or address book", then "Additional Address Books", and add the
OAB.  Then keep clicking Back until you get back to the first dialog box,
and go back to the Directories and Address Books list as you did earlier.
Once the OAB appears in the list, select it and then click Change to make
sure the Contacts folder(s) you want to display are listed.  If any of them
aren't listed, you'll need to enable those Contacts folders as Outlook
Address Books by right clicking the folder, selecting Properties, clicking
the Outlook Address Book tab, and checking the "Show this folder as an
E-mail address book" box.  Make sure you restart Outlook after making these
changes.

I experienced the same problem after a recent Microsoft
XP migration.  I had to update the list of contacts and
delete the first one that was empty, there was a second
one in the list and it became the default after I deleted
the other one.

Here's how...
Open up Outlook,
Select tools
Select E-mail accounts
Select under directory "View or change existing
directories or address books."

You may need to add your current contacts folder and
delete the default one.

Good luck,
David.