Hi rookie... ;)
Ok, first of all, no, you haven't locked yourself out of your Yahoo mail account permanently, unless the alternate email address you used when you registered is no longer valid and/or you don't remember your birthday and ZIP code when you registered. You can always tell Yahoo to email you a new password.
But, the problem is most likely simple...
1. Which version of PASSWORDMAKER are you using?
2. What Browser/version?
3. Are you using the 'Defaults' settings, or did you create an actual account for it?
4. Are you certain you are using the exact same Master Password when loggin in as you used when you created the new Password?
The problem is most likely one of these, but the most important thing isnot to panic - we will figure out what went wrong.