Have you checked the Trash folder?
Do you have any other devices that you use to read your email?
I assume that you are a current BT Broadband, or BT Premium Mail customer?
This won't help you with the current issue, but you really should think about using an email client such as Thunderbird to manage your emails. It will download new messages with a single click, enabling them to be stored them on your computer. It's more user-friendly and, properly backed-up, way more secure than entrusting your stuff to a cloud system such as webmail.