When?  Describe the symptoms more clearly.  I suspect you're confusing your
autocompletion cache with the Contacts.

One of two things is happening - either your contacts isn't enabled as an
address book or it is, but the GAL is shown first.

Try this first:
If you have Contacts in the Contacts folder but they are not accessible when
you click on the To button, check these settings:

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 is 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.

If those are both ok, then go to tools, address book, and look on the
address book's tools, options menu. Is contacts listed in the lower half? If
not add it... if you want it shown first, make the changes in the upper
part.