You have to sort that out with your email-provider.

T-bird can't login without a pw and you just changed it. So whatever T-bird had stored will not work any more.

Somehow you have to prove to them that you are who you are and that you have the right to access this email-account.

Trying to help my 85yr old mother - her password works on the iPhone and iPad, just not on the iMac. Very frustrating. I live far away and Bly here for a very short while to try and help :-(

If it works on Ipad then you can read the password-mail

Incorrect server settings will cause the password error. If you can use the password to log in from other devices and the webmail page you need to confirm that the server settings are correct.

I can't visualise the p/w anywhere else Tried to access the webpage from iPad but no prompt there to confirm re server settings...

You confirm the password at the providers webmail page by logging in with the username and password. If they work there you have the correct credentials. At that point you need to get a current list of server settings from your provider and double check what you have in Thunderbird. You can post the Thunderbird troubleshooting info here and let someone look at the settings you have now. They might spot a problem. You need to know you are using the correct username and password first.