From what you said it sounds like the browser has saved your password and is auto-filling it when you go to log in. In which case you can find your password in your browser settings.
I don't know specifics for all browsers but in Chrome settings there is a link to "all passwords" which gives you a big list of websites and the passwords saved for them. Find pokemondb.net and click the eye icon next to the password to see it.
Then if you want to change the password go to your "My Account" page (click your username at the top then "My Account" tab) and go to the bottom of the page.
If you don't have access to the original email address any more, and you are logged in, you can also change your email on the "My account" page,, after which you'll need to confirm it (an email will get sent to the new address).
Hope that helps.