Did you change it at some point from the default of 1234? If so, I think think the only way to get back to that position will be to do a factory reset. This will of course wipe any other personalised settings, delete saved logins, added apps etc.
From the 29th of March our TalkTalk Community will move to peer-to-peer support and the TalkTalk team will no longer be here to support with queries. Don’t worry we still have plenty of ways to reach out if you need to speak to TalkTalk directly. For more details check out the link here:
How to contact TalkTalk Broadband - Help & Support.
