Without knowing who the mail provider is, and the details of any error message you see, I can do nothing but guess.

I will guess your provider has issues today, or changed the settings required to connect, or your antivirus updated and choked on it's breakfast so mail does not work.

Login to server imap.virginmedia.com with username alan.green27@ntlworld.com failed.

logging into virgin media only gives the messages since thunderbird login failed. Why have these been blocked? Can you please unblock them, what was the reason?

Does this mean I can never use Thunderbird again even if I uninstall & reinstall? And I have just discovered that I can see all of my past messages etc on my android phone in Outlook , so it is not a complete disaster.

Please understand, Thunderbird is only a messenger. It sends account information to the server and reports back any messages from the server. Thunderbird does not block or in any way attempt to manage the account. If your password isn't working, you might contact virginmedia. I've seen several problems from people lately with virginmedia accounts, and don't know if there is something systemic or just coincidental. A @matt indicated, when email stops functioning, it is usually something external to Thunderbird, such as antivirus or firewall or email provider's server. Since a glitch may have happened, I suggest reviewing your account settings and your password to be sure they are correct.

You say it is not a password thing but it is the thunderbird password that is the problem. The prompt for the password comes up immediately before I have a chance to do anything else. I can still get virgin media e-mails direct from their e-mail account, but only messages since the thunderbird block. I have changed no passwords. What I do not understand is how I can get all my previous e-mails on my android phone. Is it because there is no thunderbird app there?

You can verify this by - clicking settings>privacy&security - click savedpasswords and then showpasswords - highlight the rows for your account - one incoming and one outgoing - and delete - close and restart TB - now you have no passwords filed and should be prompted for the correct password. If it does not work, that indicates that Viirginmedia prefers unique passwords for email clients (which is common for many, such as gmail, AOL, verizon, comcast, and others) and you would need to contact virgin media website for an 'app password' to use.

No saved logins are shown.

OK I have been a dimwit, I totally misunderstood what was happening. The problem was that a password had never set up for virgin media e-mail and one was expected. So I set one up and now everything works!

I'm pleased that all is well. :)