When you have Contacts in the Contacts folder but they are not accessible when you click on the To button, you need to 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 and you use an older version of Outlook, 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 you use Outlook 2007 you can’t remove just the Outlook Address book service, you need to remove the email account from your profile and recreate it. In many cases, removing the account and recreating it will fix the problem, but in a few cases you will need to make a new profile.

To remove the email account, close Outlook. Go to Window’s Control Panel and find the Mail applet (use Control panel’s Classic view) and remove the account from your profile, then add it back. Restart Outlook. If the address book is still missing, close Outlook and go back to the Mail applet and make a new profile.