There can be several reasons for this issue. One common cause is that the contacts folder may not be marked as an address book. Another possibility is that the address book may be disabled or hidden. It could also be due to a corrupted Outlook profile or a software glitch.
The first step is to check if the contacts folder is marked as an address book. To do this, right-click on the contacts folder and select "Properties". Then, go to the "Outlook Address Book" tab and make sure the option "Show this folder as an e-mail Address Book" is checked. If it's already checked, try disabling and re-enabling it. If that doesn't work, try restarting Outlook in safe mode or creating a new Outlook profile.
If only a few contacts are missing, it could be because they are not marked as electronic contacts. To check this, open the contact's details and make sure their email address is entered in the "E-mail" field. If it's not, enter the email address and save the contact. If all contacts are missing, it could be due to a disabled or hidden address book.
To enable a disabled address book, go to the "File" menu and select "Options". Then, go to the "Contacts" tab and click on the "Address Book" button. In the "Address Book" window, click on "Tools" and select "Options". From there, you can enable the address book that is not showing up. If it's hidden, you can unhide it by going to the "View" tab and checking the box next to "Show as Address Book".
If you have accidentally deleted or lost any contacts, you may be able to recover them by going to the "Deleted Items" folder or using the "Recover Deleted Items" feature in Outlook. If that doesn't work, you may need to restore your contacts from a backup or use third-party recovery software. It's always recommended to regularly backup your Outlook data to prevent loss of important information.