If you previously had Thunderbird auto connecting because it has stored the password, then it will be visible here on your old computer: 'Menu icon' > 'Options' > 'Options' > 'Security' > 'Passwords' tab click on 'Saved Passwords' button click on 'Show Passwords' button. If this is no longer possible to locate: Thunderbird is an email client and it uses the password you set up via your webmail account accessed using a browser. So try to logon to your webmail account. There should be a 'Forgotton Password' or similar link. You need to follow the instructions to get access to your webmail account. Then you can update the password. After doing this, log out of webmail and logon again to check the pasword works. Then use that password when creating the existing mail account. Select the checkbox for Password Manager to remember the password.