Thank you very much for your answers to my questions. It has helped with regard to my understanding.
However, I think my issue is that while I called it a secondary email address it was not a NTL or Virginmedia address. I recall now that I generated it through a subscribed website that I used for business. Although I stopped subscribing to that website it remained active and I could access it and also until a couple of days ago the domain email I used that was linked to my business was active.
I have since found out that the website company was sold to another company and it seems that after a long period of time they have stopped my access to both website and email address. I guess the purpose is to get me back subscribing. However, I await a response to an email I have sent them to clarify this.
Thanks again for your assistance, there was other information there that was useful to me.