What version of Outlook are you using?

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 operating system. Use the following steps to reset
the connection. 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.

For 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.

For Outlook 2002/2003:
Make sure the Contact folder is enabled as an email address book. Right
click the Contacts folder, choose Properties then Outlook Address Book. Is
the box to enable as email address book checked?
If this is grayed out...Go to Tools | Email Accounts, choose View or change
existing directories or address book. Is the Outlook Address Book present?
If it isn't listed, add it and close and restart Outlook.  If it is listed,
then remove it and close then restart Outlook and repeat these steps to add
it.

More details available here:
http://support.microsoft.com/default.aspx?scid=kb;en-us;287563&Product=ol2002