I have had a Thunderbird account for years

There is no such thing as a "Thunderbird account". You've set up an account with an email provider, e.g. Gmail or Yahoo. Then you use Thunderbird to access mail for that account on your email provider's server, as well as to send mail via your email provider's server.

Who is your email provider?

Try to verify your credentials using webmail.

Thank you for your reply and help, I had no idea that was the case!

My internet provider is Virgin Media, who I think use Google for their email service. I have a GMail account and an Outlook account and I have tried both passwords and neither work.

Could you explain what you mean by using webmail to verify my credentials, please? I am afraid I am not as IT literate as some which I suppose is not helping.

"Using webmail" means going to the provider's website and logging in there. It's a simple check that you are using the correct login and password. In some cases you'll get a prompt that says you need to confirm something and this itself can be a reason for Thunderbird failing to connect.

Unfortunately, it's become a bit more complicated. The password you use to access Gmail via Google's website isn't necessarily the password that Thunderbird needs to use. For instance, one option in Gmail is the "application-specific" password.

I use OAuth2 with Gmail and it works seamlessly. A token is issued when you set up the account in Thunderbird, and Thunderbird uses that thereafter.

A useful first step is to remove from Thunderbird all references to the troubled account's login, forcing Thunderbird to prompt you for the password. If it has more than one password stored for a particular account , it doesn't know which to use so cannot proceed automatically. Clearing multiple passwords gives it a clean start.

If you're running any security software it might be interfering with the authentication process. Try running Windows in Safe Mode to temporarily disable your security software as a way to check if this is causing you trouble.