if you still have the thunderbird that the IT person set up, you can try this: - click settings>privacy&security - then click savedpasswords and then showpasswords - from there, you can copy the password. You may be prompted for your Windows signin password prior to thunderbird showing the passwords. AND, that saved password won't work on the new device if your email provider requires special 'app' password on each separate device (Gmail is an example of this).

Thank you for responding, but problem is NOT solved..

I understand. If the saved password does not work, then you need to contact your email provider for a new password. All that Thunderbird does is store the password provided to it and then send to server whenever an email is sent.