Have you previously set up Password Recovery for the Tiscali email address? You could use the Reset your Password option. Or if you're a TalkTalk broadband customer or recent customer you can change your email password in MyAccount > My Services > Webmail - Manage Webmail, if the email is listed in one of the 5 email address slots.
If not, you'll need to prove you're the legal owner of the email so you can get new password. Double check that you've added to your Community Profile, Personal Information (Click here) all of the following in the Private Notes area at the bottom of the page:
This topic has been escalated to the TalkTalk Community support team to verify the details recorded for the owner of the email address. The OCE will reply here to help with account or password recovery.
...Ah, yes if the mailbox is over the limit the account may just lock up.
One of the networks team will need to take a look to see if they can recover access. If the mailbox is over full then some emails will have to go to allow the email to work. Can they just delete some of the oldest emails from the Inbox to get the thing functioning again? What do you think?
If the mailbox has got corrupted they'll try running a repair job and if this provides access then you could delete some to get it functioning again, maybe download a batch of emails and store them locally.