Have you tried to access your email account on the web mail site first? That would be the best place to start. If you can access there, then you know your password.
Then go to Thunderbird and try sending an email. If the password isn't correct, you get a small dialogue box asking for the correct password, with a tick box underneath to tell the program to remember it.
That's the way to change your password generally, having changed it on webmail beforehand.
You can get to view your passwords quite easily by going to Tools - Options - Security (tab) - Saved Passwords - Show Passwords (bottom right of new window)
Hopefully you will now be able to access your emails on Thunderbird :)