Grateful if you'd confirm if this is the Address Book / Contacts associated with your TalkTalk Webmail and confirm if your webmail has been upgraded to the new TalkTalk Mail platform. You'll know because you'd have received an email from TalkTalk advising Upgrade complete.
Please ensure your Community Profile Personal Information (Click here) includes your name, address, current TalkTalk landline 'phone number and an alternate number (mobile recommended). Add, then scroll down and add your email address in Private notes and save changes. The TalkTalk Community Team OCE will then be able to link to your TalkTalk email account and if it is a TalkTalk issue will raise a fault ticket.
I've not been made aware of the reason in your case but based on the other reports here it may be the system locking the Address Book whilst carrying out an upgrade.
Is the Address Book now back in action for you? Have you received any messages from TalkTalk about upgrading your mail service to the new TalkTalk Mail platform?