Email is not the place to store important documents, its best to save them to a secure storage device, or print a hard copy.
Its a pity you were not using an email client, as that would have given you the option of backing up any messages.
If they have been totally deleted by you, or one of your devices, them at least nobody else can access them. I am assuming that your account has not been compromised, and someone else has downloaded your information.
There are lots of phishing attempts by people pretending to be BT, and telling people that BT mail is changing, and they have to login to confirm their details, otherwise their account will be deleted.
