Password for what?

If it is for your e-mail account, you must re-set it through your e-mail service provider. It is not a Thunderbird issue.

But you can get your password off your old Thunderbird profile if you still have access to it. Do you?

If you have Thunderbird on an older PC that works for your account, you may be able to copy the profile to new computer or at least retrieve password from old computer. If not, I suggest you contact your email provider.