your email address when accessing MYBT is just a username and nothing to do with actually accessing your email account if you have not been using your BT email i.e. checking emails from friends/family etc then it sounds like it has been deleted
Using your email address to log onto MyBT is not the same as using your email account. When you use it to log on all you are doing is using it as a user name not an email account.
You needed to log on to your actual email account where you check for and send emils from. If you did not do that the email account gets deleted and can not be recovered to be reused.
The email address will still work as a username so you can still log onto your MyB with it.
Why do BT's customers not read the Terms and Conditions which tells them about using an email account and what will happen if they don't.
What has been said in this thread is correct, you can find more information here - Why has my BT Email address been suspended or deleted?
If you email hasn't been accessed for 90 days then its more than likely been suspended. You would still have 60 days to recover that email address. If it gone passed that 60 days then I'm sorry as nothing can be done to recover that email.