I am assuming you are talking about a Yahoo profile. In order to do that, you are going to need to contact Yahoo.

I suggest you go to http://groups.yahoo.com . There will be a login screen. If you try to log in to Yahoo using that profile, there is a link that will say something like "forgot your password"?

Usually you can follow the instructions from there. The only catch is if you changed your email address, it's more involved.

Good luck!

Kitty