There is a fault accessing your TalkTalk MyAccount at present so changing the email password via MyAccount is temporarily unavailable.
Have you tried to sign in at TalkTalk Mail and is there any error message? Let us know what that message is.
The alternate way to change a password is to use the Reset your Password option. If there's an alternate shown that you recognise from the hints, and have access to, then please reset the password by having a reset link sent to either the alternate email address or by text to the mobile number.
If the lineone email account hasn't been used for a while then it may be suspended. You'll need help to unsuspend it. If it hasn't been used for over a year then it may well be deleted.
Let us know.
In the meantime, please update your Community Profile Personal Information (Click here) to include your name, current contact 'phone numbers and address with postcode (location) then scroll down and add the email address to Private notes and Save changes.