I suspect that it is connected to you not being a BT Broadband customer and the outage that took place.
When you are no longer a BT Broadband customer the email account gets downgraded to a BTMail Basic account which can only be used on webmail.
This normally happens when a person leaves BT but there have been cases reported on the forum that a leavers email account was not downgraded, that is until they changed their password or they deleted the email account and tried to set it up again either on the same device or onto a new device all of which seemed to trigger the downgrade.
There have also been reports that it just randomly happened at a much later time after the person left BT.
When an email account is downgraded the account holder can still "manage" it through their MyBT albeit the MyBT account is basically only a holding account so that the email is "accounted" for. It is not a billing account.
Do you have a MyBT account and is the email account still showing there and is it showing account type as "standard"?
In any event I doubt that there is anything you or I or other forum users will be able to fix the problem so you will need to call BT 0330.1234.150 and ask to speak to the BTMail email team and hopefully they should be able to resolve it.
Please keep the tread updated with how you get on and what the solution is should they manage to fix it.
I just spoke to them and it is working again now, although they didnt actually do anything. they said my account shows as premium even though i do not pay for it or bt internet. I explained about the beta testing i did for them and said that it must have been due to that and i was probably granted lifetime premium as i was an original tester, (i was told this at the time that my email would always be active) he confirmed my account was active since 1999 and not 2000 (when adsl launched) and that i was originally on an HG1 account, so that confirmed i was a tester for them. he also saw i had 2 email adresses @btinternet.com and @btopenworld.com and that both were the same account. whilst i was on the phone i tested again and this time the email went through (phone to pc and then pc to phone) the person explained about the problems and said that they have had people working on it, so it must all be related to the original problem yesterday.
i will mark this issue as resolved, thank you for all the help and espeically providing te number to call as at least i know why i am not limited to using webmail etc. over the 26 years i have setup on multiple new pc's/phones without issues.
Glad to hear its sorted and thanks for the update.