There was a BTMail email outage yesterday. It has been resolved now and things should be back to normal provided you did not do anything such as deleting the email account or trying to change the password as doing that when there is a problem always causes more problems than it solves.
If it is still not working on your iPhone try restarting the phone to see if that resolves it.
Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account?
If neither of the above your email account may have been downgraded to a BTMail Basic account which can not be used with an email client
See link
https://www.bt.com/help/email/bt-email-products
