I'd first check with BT if they have problems.

ianmw77 said

As of this morning, Thunderbird is suddenly unable to log on to btinternet to get my mail and keeps asking for a new password. However, the password that it has is correct, and I can log on fine using a browser. Anybody know what's going on? is it BTInternet (most likely) or Thunderbird? It's logging on fine to other accounts.

Eventually BT solved the problem after an hours messing about. It's nothing to do with them at least. There seems to be a problem with passwords on Thunderbird expiring, and not telling you. Changing the pword on the email account, then changing it on Thunderbird brought everything back to life. The question now is - why doesn't TBird offer some sort of control over expiring pwords, or at least warn you about it?

why doesn't TBird offer some sort of control over expiring pwords, or at least warn you about it?

Thunderbird has no visibility when an account password expires. Passwords are maintained by your email provider on their servers (where your account is). Thunderbird just saves a copy of your password locally, so that you won't need to enter it manually every time you access your account. Login to your account via webmail every once in a while and see if you get a notification next time.

I know that's the theory, but it doesn't fit the facts. Why, when I change my email password, and then change the TBird password does everything come back to life? And all the time I am able to get to my email account by using a browser, but not TBird. Something is wrong, and noone seems willing to own up to what.

Bt/Yahoo have has some serious outages in the past couple of days with mails disappearing and a host of other damaging rot.

Usually if Thunderbird tries to connect with a bad password it get an error from the server and prompts your for a new one. If the server is all messed up that what happens is anybodies guess. In this case no one is entering into prolonged discussion/ diagnostics on anything Yahoo or BT until they get their collective act together and fix whatever is broken.