You say it was a letter and not an email or even telephone call?.
I had problems with TalkTalk once, over their mistakes in setting up a direct debit payment arrangement. The only thing that I know about a non-payment, was when I find that my broadband but not my telephone had been terminated immediately. A telephone message about contacting TalkTalk was left on an answer machine, but I assumed this was for a sales pitch, because TalkTalk wanted me to upgrade.
I would suggest that you contact TalkTalk accounts via their landline support telephone contact details, assuming that you are indeed a TalkTalk subscriber. I did, and the issues were resolved, there and then.
No point in 'thinking' what might be happening, then perhaps find the letter is genuine (with mistakes), then finding you have been 'cut-off'. Because as I said, from my previous experience, TalkTalk give very little leeway to account problems, even though it might be on their side, the mistake was made?.