Thunderbird doesn't keep passwords, other than the ones you provided for accounts you set up. Otherwise, you will need to contact your email provider.

Do you still have access to Thunderbird on your old PC?

Yes, I do

You might retrieve the password from the old PC if it is the same email address and if you saved the password. Settings > Privacy & Security > Passwords > Saved Passwords.... button etc

I'ts not in there. I'm going to go to mozilla T-bird and see if I can reset it. Thanks

"Mozilla T-Bird" is an email client, no hosting service. You must contact the provider where you created (or were given) the email address in question.